Information in the tail may be a reference to a specification or code, a welding or cutting process or special instructions / directions. With that said, "BC" does not ring a bell. Just a guess, not knowing what type of joint you are asking about, could it be a typo for "BG" (backgouge)?

Unfortunately, you might have to go back to the detailer to know what the "B.C." actually means. Most all other parts of the welding symbol have a standard format to follow but the information in the tail does not have to follow any particular standard. And some detailers are known for wild imaginations (not all, just some :)
Sometimes a company will adopt abbreviations that everyone in their employ will understand. So you might also contact whatever company the drawings were made for.

Chet Guilford