The C/P column is short for Calculation basis / Position. This is where you select a processing code for a calculation basis. The first row is always a position row. The position number is automatically set to 1. If you are going to insert a new position row instead of a processing row you should enter a new position number here, such as 2.
The calculation bases that you can select depend on the program licenses you have. You will find a description of the bases that exist in the system and their respective input data and variables under Calculation Basis.