This module allows you to select related terms in taxonomy from another vocabulary. The default taxonomy module only allows you to select related terms from the same vocabulary.
You setup the vocabulary relationship in the "Settings" fieldset of the "Edit Vocabulary" screen. Once this is done, you will see another "Related terms in %vocab-name" under the "Advanced options" fieldset on the "Add term" screen. This module uses the existing term_relation table used with taxonomy module. You can then use the taxonomy_get_related() function from the taxonomy module to find all related terms to the passed in term id.