A key feature of XBRL is extensibility, allowing implementers significant freedom to build mould solutions to specific requirements. In order to ensure that such extensions are done consistently, and to avoid unnecessary reinvention of similar extensions, XBRL provides a number of registries.

These registries provide a centralise list of definitions, allowing implementers to re-use suitable definitions created by others. Although use of the registries is not compulsory, re-using definitions wherever possible will promote interoperability.

In addition to the registries listed below, the Inline XBRL specification is supported by the Transformation Registry 2.