required thickness of a pipe (asme B31.3, 304.1.1)

Asme B31-3 (304-1-1) says:
Tm=t+c

where c is the sum of the mechanical allowalnce (thread or groove depth) plus....


question1: what is groove depth?

question2:if we have a straight pipe having BW endings, have we to consider mechanical allowance or not?

question3: using formula (3a) of (304.1.2) there is no "c" inside formula so we obtain t and tm=t+c ,formula (2) . But is we use formula (3b), inside formula there is "c", so is formula (2) to be read as tm=t or is the same tm=t+c?

thank you