We usually call uniform A as the BSA official uniform while uniform B is casual T-Shirt designed by our troop and the style varies from year to year. 

We wear class A uniform in Troop Meetings and Flag Ceremonies during campout. Scouts also need to wear class A uniform when doing scoutmaster conference and Board of Review.

For other activites such as campouts, hiking, service projects, etc., we wear class B uniform.

Below is the official BSA insignia guideline:

You may refer to some other high level versions for easy and quick understanding: