OTC is by zone and zones are usually made up of 2-4 individual hunting units. For example the Lemhi zone consists of units 29, 37, 37A, and 51. A Lemhi zone "A" tag means you can hunt where ever you want in those units Aug 30 - Sept 30 for any elk with archery equipment. If you don't tag out with archery equipment it be used as an OTC muzzy tag for cows Nov 25 - Dec 9 in those same units. Also it can be used as an OTC spike tag in unit 37 Nov 1-7.
Specific unit tags are mostly for the draw. That same zone has no OTC rifle bull tags, so you'd apply to draw a rifle tag for the specific unit you want to hunt in.