James,
They can do both of those if your vice is big enough.
What they are basically is what the name says, an indexer.
If you can divide 360 by a number and end up with a full number, so that is how many sides can be machined, or holes drilled or teeth cut etc, etc, around the circumference of a bar.
360÷12=30, so you can have 30 teeth on a gear, or even machine 30 flats on a bit of round bar, or in theory 360÷1=360 ......................
OR
If you need to drill holes at say 0, 37 and 193 degs around a circumference, then that can be done as well.
Basically, whatever you can think of for a use of full degree measurements around a circumference, then it can be done. The more you use your imagination, the more use you can get out of it.
This is a post I did about cutting the foot down to fit my vice and putting a t-slot bar along the bottom.
http://www.homemodelenginemachinist.com/index.php?topic=2681.0Bogs