Possible formula variables in salary structure

Dear friends,
We know that in conditions we can mention many variables to check, like nationality, payment_days, total_working_days etc.
but what are the possible variables that we can use in side a formula? We know that abbrivations of components can be used but what else is possible? for example how we can use payment_days in formula?

You can only use abbreviations of the salary components.

In the Salary Slip, Payment Days are auto-calculated based on days in the month, Leave Applications of the Employee and Leave without pays (LWP). Based on LWP, amount of salary is also auto-recalculated.