With FirstSpirit 4.2 external links could be grouped in categories:
All links inherit the category of their link template (at creation time).
There are only two predefined categories ("url" and "email") but your can define your own categories.
The categories could be used as filter in the api method "getExternalReferences". The call below collects all external references with the catogory "email":
ReferenceEntry[] email_references_release = context.getProject().getExternalReferences("email", true);
René has posted a use case where categories come in handy: Periodically check of external urls.
Tested with FS4.2R2