IMS and IEEE use this word in the context of metadata, to describe an element that may contain sub-elements.
An example of an extensible element is the IEEE LOM element "1. general". This element has sub elements including "1.1 identifier" and "1.2 title". IEEE LOM only allows extensions that are defined in the draft standard to be used in a compliant implementation.
The IMS Learning Resource Meta-Data Specification allows implementors to extend elements to suit their needs, by using the XML binding to define such extensions in a uniform manner.
IMS are seeking consensus on the use of extensions, and point out that using extensions may break interoperability, which the specification aims to promote. IMS also request that any extensions of metadata elements ought to be brought to the attention of the IMS Meta-Data Working Group. This may be done through the CETIS Metadata SIG.
The extensible nature of some IMS/IEEE metadata elements is not a feature of Dublin Core, which uses a flat structure for its metadata elements. Extending metadata elements has a similar function to the refinements in Dublin Core, however.
IMS Global Learning Consortium
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