Piping : Rf Pad Thickness Calculation

Hai..
I am new to this forum. So let me introduce myself. I am richard, working as piping materials engineer.
I have a query regarding the reinforcement pad thickness calculation.
As per ASME B 31.3 (2004 edition), section 304.3.3 Reinforcement of Welded Branch Connections, the diagram for RF Pad thickness calculation has shown notations like t_b, t_h,T_b,T_h, C, Mill tolerance etc..
Now T_b or T_h refers to the t_min which is the minimum pipe thickness of branch and header respectively which is required.
When mill tolerance is added to T_b we get Nominal pipe thickness.

Now my question is, as per my knowledge, the ' t_b or t_h ' shown in the diagram is the thickness calculated by considering the internal pressure only.
And 't_b or t_h' + 'corrossion allowance' gives minimum pipe thickness, which is called T_b or t_min, what ever it is.. Now tell me what does the 'gap thickness' present in diagram between 't_h or t_b' and 'C' refers to? (also mentioned as area A2 & A3 in diagram).

Hope the readers understood the question.
I am a guy of just 2 months experience. So I dont know whether i am asking a right question, but if i am wrong then plz excuse me.