# The Structure of LOM > This documentation does not warrant completeness or correctness. Please report any missing or wrong information using the [ILIAS issue tracker](https://mantis.ilias.de) or contribute a fix via [Pull Request](../../../../docs/development/contributing.md#pull-request-to-the-repositories). Metadata of objects in ILIAS follow the Learning Object Metadata (LOM) standard (for the most part, see [here](#specific-to-ilias) for diversions). The standard will not be reproduced here in full, this documentation is restricted to providing information useful for working with LOM in ILIAS. LOM consists of a nested set of elements, forming the structure illustrated below. The elements in the set can not be uniquely identified by name alone, but by their position in the set: they need to be selected by the path to them from the root element `lom`. All elements but the root element `lom` are optional, meaning that in any given LOM set they might not occur at all. Elements marked with `*` are unique, at their position in the set they may not occur more than once. Of non-unique elements, arbitrarily many instances can occur. The leaves of the set carry data of different types, which will be explained below. ```` lom* ├── general* │ ├── identifier │ │ ├── catalog* (string) │ │ └── entry* (string) │ ├── title* │ │ ├── string* (string) │ │ └── language* (lang) │ ├── language (lang) │ ├── description │ │ ├── string* (string) │ │ └── language* (lang) │ ├── keyword │ │ ├── string* (string) │ │ └── language* (lang) │ ├── coverage │ │ ├── string* (string) │ │ └── language* (lang) │ ├── structure* │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ └── aggregationLevel* │ ├── source* (vocab_source) │ └── value* (vocab_value) ├── lifeCycle* │ ├── version* │ │ ├── string* (string) │ │ └── language* (lang) │ ├── status* │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ └── contribute │ ├── role* │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ ├── entity (string) │ └── date* │ ├── dateTime* (datetime) │ └── description* │ ├── string* (string) │ └── language* (lang) ├── metaMetadata* │ ├── identifier │ │ ├── catalog* (string) │ │ └── entry* (string) │ ├── contribute │ │ ├── role* │ │ │ ├── source* (vocab_source) │ │ │ └── value* (vocab_value) │ │ ├── entity (string) │ │ └── date* │ │ ├── dateTime* (datetime) │ │ └── description* │ │ ├── string* (string) │ │ └── language* (lang) │ ├── metadataSchema (string) │ └── language* (lang) ├── technical* │ ├── format (string) │ ├── size* (non_neg_int) │ ├── location (string) │ ├── requirement │ │ └── orComposite │ │ ├── type* │ │ │ ├── source* (vocab_source) │ │ │ └── value* (vocab_value) │ │ ├── name* │ │ │ ├── source* (vocab_source) │ │ │ └── value* (vocab_value) │ │ ├── minimumVersion* (string) │ │ └── maximumVersion* (string) │ ├── installationRemarks* │ │ ├── string* (string) │ │ └── language* (lang) │ ├── otherPlatformRequirements* │ │ ├── string* (string) │ │ └── language* (lang) │ └── duration* │ ├── duration* (duration) │ └── description* │ ├── string* (string) │ └── language* (lang) ├── educational │ ├── interactivityType* │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ ├── learningResourceType │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ ├── interactivityLevel* │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ ├── semanticDensity* │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ ├── intendedEndUserRole │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ ├── context │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ ├── typicalAgeRange │ │ ├── string* (string) │ │ └── language* (lang) │ ├── difficulty* │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ ├── typicalLearningTime* │ │ ├── duration* (duration) │ │ └── description* │ │ ├── string* (string) │ │ └── language* (lang) │ ├── description │ │ ├── string* (string) │ │ └── language* (lang) │ └── language (lang) ├── rights* │ ├── cost* │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ ├── copyrightAndOtherRestrictions* │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ └── description* │ ├── string* (string) │ └── language* (lang) ├── relation │ ├── kind* │ │ ├── source* (vocab_source) │ │ └── value* (vocab_value) │ └── resource* │ ├── identifier │ │ ├── catalog* (string) │ │ └── entry* (string) │ └── description │ ├── string* (string) │ └── language* (lang) ├── annotation │ ├── entity* (string) │ ├── date* │ │ ├── dateTime* (datetime) │ │ └── description* │ │ ├── string* (string) │ │ └── language* (lang) │ └── description* │ ├── string* (string) │ └── language* (lang) └── classification ├── purpose* │ ├── source* (vocab_source) │ └── value* (vocab_value) ├── taxonPath │ ├── source* │ │ ├── string* (string) │ │ └── language* (lang) │ └── taxon* │ ├── id* (string) │ └── entry* │ ├── string* (string) │ └── language* (lang) ├── description* │ ├── string* (string) │ └── language* (lang) └── keyword ├── string* (string) └── language* (lang) ```` ## Data Types ### String (`string`) String-data can have as its value any string, except an empty one. ### Language (`lang`) Language-data can can have as its value any of the following two-character codes, including `xx` as a stand-in for the token `none`. ```` aa, ab, af, am, ar, as, ay, az, ba, be, bg, bh, bi, bn, bo, br, ca, co, cs, cy, da, de, dz, el, en, eo, es, et, eu, fa, fi, fj, fo, fr, fy, ga, gd, gl, gn, gu, ha, he, hi, hr, hu, hy, ia, ie, ik, id, is, it, iu, ja, jv, ka, kk, kl, km, kn, ko, ks, ku, ky, la, ln, lo, lt, lv, mg, mi, mk, ml, mn, mo, mr, ms, mt, my, na, ne, nl, no, oc, om, or, pa, pl, ps, pt, qu, rm, rn, ro, ru, rw, sa, sd, sg, sh, si, sk, sl, sm, sn, so, sq, sr, ss, st, su, sv, sw, ta, te, tg, th, ti, tk, tl, tn, to, tr, ts, tt, tw, ug, uk, ur, uz, vi, vo, wo, xh, yi, yo, za, zh, zu, xx ```` Note that ILIAS is more restrictive than the LOM standard, as the standard also allows for example three-character codes, or adding hyphen-separated suffixes to the base codes. ### Vocabulary Value (`vocab_value`) The value of vocabulary value-data should be set to one of the values in a vocabulary, pre-defined for each element with this data type separately. These vocabularies can be configured in the ILIAS Administration, for more information see [here](vocabularies.md). Note that elements with other values than those from the element-specific vocabularies are not treated as invalid by ILIAS. This is to avoid information being lost on import/export between installations with differently configured vocabularies, or when re-configuring vocabularies. ### Vocabulary Source (`vocab_source`) Vocabulary source-data should be set to the source of the vocabulary that the associate `value` element is from. For more information see [here](vocabularies.md). As described [above](#vocabulary-source-vocab_source), elements with values not conforming to this are not treated as invalid in ILIAS. ### Datetime (`datetime`) The value of datetime-data follows a specific format, simplified in ILIAS to `YYYY-MM-DD`, since ILIAS only processes the date portion of datetimes. ### Non-Negative Integer (`non_neg_int`) Non-negative integer-data can have as their value any combination of any number of digits. ### Duration (`duration`) The value of datetime-data consists of up to six non-negative integers, arranged in a specific format: - the number of years, appended by `Y` - the number of months, appended by `M` - the number of days, appended by `D` - the number of hours, appended by `H` - the number of minutes, appended by `M` - the number of seconds, appended by `S` All of these fields are optional. If any of them are present, the value is prepended by `P`, and if any of the last three are present they are prepended by `T`, e.g. `P12Y55MT35M`, `P0D`, or `PT9999S`. Note that the LOM standard also allows for fractions of seconds, but this is not implemented in ILIAS. ## Further Restrictions ### From the Standard If at least one element `metaMetadata > metadataSchema` is present, it must have the value `LOMv1.0`, meaning the first occurence of this element cannot be edited or deleted directly, and comes pre-filled (it can however be deleted by deleting `metaMetadata`). ### Specific to ILIAS Since the value of the element `general > title > string` is synchronized with the title of the ILIAS object the LOM set belongs to, neither it nor its super-elements can be deleted. The sub-elements of the first `general > identifier` are fixed and can neither be edited nor deleted, along with their super-element. The `catalog` sub-element is always set to `ILIAS` and the `entry` to `il_{Installation ID}_{Object Type}_{Object ID}`. `Object ID` refers here to either the `sub_id` if it is non-zero, and otherwise to the `obj_id` of the object as defined [here](identifying_objects.md). The LOM standard allows any elements of type `LangString`, meaning those with a tupel `string` and `language` as sub-elements, to have arbitrarily many such tupels. This is currently not implemented in ILIAS. Further, some elements denoted above as having data type `string` are more restrictive in the LOM standard. For example, `technical > format` should be restricted to MIME types (or the token `non-digital`), and `contribute > entity` should be restricted to representation of vCards. Likewise, these restrictions are currently not implemented in ILIAS.