In S.1.4 it is précised that the minimum thickness in this appendix do not contain any corrosion allowance.
In S.3.2. the formulas add the CA to determine td.

If the minimum thickness is determined according to 3.6.1.1. the corrosion allowance allowance shall be added to this thickness or not.

The equations in Appendix S allow for a "corrosion allowance", if specified by the purchaser.

For Section 3.6.1.1, the typical procedure is to determine the minimum thickness, then add the corrosion allowance. Then, if this value is less than the value in 3.6.1.1, the value from 3.6.1.1 is used instead.