B can indicate one of three different shapes. When it is used by itself, though, it stands for bullet. A bullet light bulb is a cylinder with a rounded end, like a tubular incandescent bulb, but with a base that is smaller than the glass cylinder.
A BR bulb is a bulged reflector bulb. BR bulbs typically have two funnel-shaped reflector areas, one behind the other, to help shape and focus the light.
A BT code may indicate a bulged tube or chimney light bulb, but it can also stand for a blunt tip light bulb—the ones that are flame-shaped with a rounded end. On the other hand, to be totally confusing, BT light bulbs can also stand for bent tip, which is the exact opposite of a blunt tip. To minimize the confusion, many companies just spell out blunt tip or bent tip on the package rather than relying on the code.
Source: The Spruce