Salary Rules are the various types of Allowances, Deductions, etc.You can define Salary Rules under salary structure for the easy computation of monthly salary.

Payroll module no more in odoo13 CE, So request to use free module from odoo Apps.

Link: https://apps.odoo.com/apps/modules/13.0/hr_payroll_community/

There is already describe the details how you can create/update new salary rules.

# Available variables:

#----------------------

# payslip: object containing the payslips

# employee: hr.employee object

# contract: hr.contract object

# rules: object containing the rules code (previously computed)

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

# worked_days: object containing the computed worked days

# inputs: object containing the computed inputs.

# Note: returned value have to be set in the variable 'result'

result = rules.NET > categories.NET * 0.10