Bass Management
Joebob,
The bass management settings do overlap somewhat, but here is the distinction. The SMALL/LARGE setting relates to the audio track for each speaker and enables/disables the crossover. The SUB, FRONT, BOTH setting relates to the LFE track and allows you to route this signal to the fronts if you don't have a sub, etc. By using the BOTH setting, you are sending all of the LFE content to the fronts and sub, not just the info from the fronts.
For most speaker configurations, speakers set to SMALL, and LFE to sub are the best configuration.