Collecting units toward your win condition unlocks elite levels at 3/9, 6/9 and 9/9 for each of those units. To see what bonuses they receive, hit the V key to open up the Victory Information window.

Some more quality of life stuff and polish will come in future patches, but I wanted to push this patch out since the bonuses now work for every unit. Also in future patches, enemy units will randomly be promoted to Elite versions of themselves.