Raiders and Cowboys essentially swap fullbacks and draft pics. Keith Smith the Dallas fb last season signed as a free agent with the Raiders after being non-tendered by Dallas, so Dallas just traded for fb Jamize Olawale and the Raiders 6th round pick 192 overall), for a 5th round pick (172 overall).
A lot of the designed run plays in the playbook in the Dallas playbook are dependent on the fullback for execution and Elliot's numbers are quite good on plays with a fullback, so Dallas needed to replace Smith with a competent blocking fullback. An added plus is that Olawale is a superior blocker in pass protection too.
Small moves like this often get overlooked in the off-season, but pay significant dividends over the course of a season, but since fb never get fantasy points, most football followers gloss over it.
-M