This schema specifies the format for lists of allowable relations. Lists occur in special-purpose documents containing nothing else, that are made accessible on the internet. BrainML links documents may refer to these by use of an 'xlink:role' attribute that is defined in the main brainmetal schema. They may NOT define relations themselves. If terms need to be added, a schema authority should submit a relation definitions document to the schema repository. Relation definitions are a bit simpler than term definitions -- there is no domain, and hierarchy is available but not often used. A 'relation' is an atomic descriptor that exists in an ISA hierarchy and is associated with a particular domain of applicability. One or more 'external-equivalent' children may exist to link the term to equivalents in non-BrainML systems, such as the UMLS. Link to internal equivalent relation. Link to external equivalent relation. This is the identifier by which the relation will be referenced. Valid characters in identifiers are ONLY letters, numbers, underscore, period, and dash, and an identifier cannot start with a number. The identifier for a relation should be unique within its containing document. It is not fully decided whether the ISA hierarchy is a tree or a DAG in which a relation may have multiple parents. In the latter case, the relation actually would occur in two or more different places in the XML element hierarchy with the same name but different IDs. One or more 'internal-equivalent' children is used to link each instance of the term to all the others. This approach is similar to that used in the HDF5 XML format to represent directed graphs. (see http://hdf.ncsa.uiuc.edu/HDF5/XML/design-notes.html) Used to link a relation with an equivalent term or definition in another system, such as the UMLS. BrainML does not explicitly support structured term definition, but through linking to another system that does, existing semantic structure can be maintained when metadata is expressed in BrainML. Top-level element for a relations vocabulary document.