B is a VM with a burstable CPU. The VM CPU has a baseline and provided that you have credits you can use more than that baseline if you need. You earn credits by having periods when your VM is bellow that Baseline.
Depending on the arcitecture of your VM and the credits you have Microsoft quote that as much as a 2000% burts in CPU performance is possible. I can't comment on that in the real world. I have never used this sort of VM.
This could be useful if you have a VM that needs to be on all the time but most of its usage is for batch processing, or if you have a mostly quiet website that occasionally sees high spikes of traffic. Personally I don't like the complexity of knowing what you're going to get that you have here so I prefer to architect my solutions with the required head room and if I need to be more cost effctive power my VM's on and off to save money.
If you see constant high usage in the VM's CPU the VM will be restricted to the baseline, which would make it not very cost effective.
As I saisd earlier, I don't favour this sort of workflow, so haven’t gone into the workings of the credit system in great detail. You can read more about how it works here