Here is what TANK is doing:

You select either "Site Specific" or "Mapped".

For "Site Specific", you're also supposed to define Sa* and Sa0*. These values are then used to determine Ai and Ac per E.4.6.2.

For "Mapped", the software checks the input values of Ss, S0, and S1. If any of these are zero (i.e. not defined), then Section E.4.3 is used to determine Ss and S1. E.4.6.1 states that Sp can be used directly for S0. Once S1 and Ss are known, E.4.4 is used to obtain Fa and Fv. Now everything necessary to compute Ai and Ac per E.4.6.1 is available.