I also do not understand relational segments.
Here is a link to a test page in OpenACS which uses a rel_segment:
rel-segments-test.sql
A relational segment creates a subgroup where all members have the same rel_type to the original parent group.
Example from the above:
-- the segment of all parties that are blah members of G
seg_G_blahs := rel_segment__new(null,
''rel_segment'',
now(),
null,
null,
null,
null,
''Blahs of Group G'',
G,
''blah_member_rel'',
null
);