What does BW mean in the BW elbow?

What does "BW" mean in the BW elbow?

Is the BW elbow welded or seamless?

The answer is that BW has no connection with welded or seamless.
It represents the welding method at the elbow's port
For pipe fittings, BW is an abbreviation of BUTT WELDED, which means butt welding. There is no inevitable connection with the welded fittings or seamless fittings. If you want to determine whether to have a weld, you need to check other descriptions.
BW butt welding (BUTT WELDING)
SW SOCKET WELDING