Speaker B is slaved to, and mirrors the source for the primary zone and its receiver. It can take a source from any input, regardless of whether it's an analog, digital, or HDMI source.
Zone 2 is a separate zone, and can have a different input than the primary zone's receiver.
You would use "Speaker B" if you wanted to set up a second set of speakers for your primary zone, such as satellite speakers in a wet bar, or if you had a 5.1 setup for watching movies but wanted set of full-range speakers to listen to music in stereo-only.
If you wanted your wet bar to be able to listen to something other than what was going on in the main zone (movie in the main zone, radio tuner in the wet bar), you would set it up as Zone 2.
My friend uses Speaker B to run speakers on his deck from his receiver in his garage. I use Speaker B to run a set of speakers in my basement from the receiver in my computer room.