The MODS Editorial Committee has considered the proposal to add a relatedItem type value for expressing the relationship between a research publication and the data set(s) that it is based on. A comparable element describing this relationship is included in MARC 21, 786 (Data Source Entry). However, it was also pointed out on the listserv that only including a relatedItem type for datasets may be too narrow to address the larger issue of different types of accompanying materials deposited in repositories and that an approach allowing for the inclusion of external vocabularies may be preferable.
The entire listserv discussion can be found here:

The  MODS Editorial Committee feels that new type values for relatedItem should be added only when they are strongly justified. Instead, we would like to introduce a mechanism whereby type values from external vocabularies can be supplied.   We propose the addition of 4 new attributes: @otherType may be supplied alone, in which case it is an uncontrolled type.
If @otherType is supplied along with @otherTypeAuth, then it is a controlled type where @otherTypeAuth is its authority.
@otherTypeAuthURI may be used to supply a URI uniquely identifying the vocabulary from which the controlled type has been selected.
@otherTypeURI may be used to supply a URI uniquely identifying the controlled type value from a vocabulary.

The MODS Editorial Committee solicits comments on this proposal. We'd like to receive feedback by August 15.

Melanie Wacker
on behalf of the MODS EC