I have a Hunter Specialties BucGrunter call. When taken apart, there is a post next to the reed with different notches for an o-ring that are, from top of the reed/post to the bottom: Fawn, Doe, Young Buck, and Mature Buck. What are the different uses for the different settings and are there different ways to blow into the call for these different uses? Thanks!

Yes, it's different sounds and different uses
Fawn: Imitates a fawn. Usually used to draw in another deer with curiosity.
Doe: Imitates a doe, used to draw in deer or attract a buck. I use it to stop a deer, as it usually always does.
Young Buck: The sound of a young buck, good for bringing a buck in closer or stopping him if he's moving.
Mature Buck: a deeper sounding grunt same use basically.

You don't really use them differently. The reed moves to adjust the pitch, you just blow into the tube.

I mostly just use the doe bleet to stop deer or bring them closer. This has proven the one that the most deer are interested in and it also doesnt seem to scare them as a buck grunt sometimes will.
Hope this helps
-Jake