There is a list of Available Variables which will be used to specify field's value(as python code) on Salary Rules.

Available variables:

• payslip: an object containing the payslips.

• employee: hr.employee object.

• contract: hr.contract object.

• rules: an object containing the rules code (previously computed).

• categories: an object containing the computed salary rule categories (sum of the amount of all rules belonging to that category).

• worked_days: an object containing the computed worked days.

• inputs: an object containing the computed inputs.

You can directly access variables from here like

1) worked_days.absense
2) contract.wage

for more info try

https://doc.odoo.com/7.0/book/4/4_11_HR/4_11_HR_payroll/