Steps:
These will be important for the reporting box (Value) on T4, T4A or RL1.
Depends by the type of category (earning, benefit, deduction, tax or payroll tax) Form assignment tab (Box number) will have a different
Below is an examples of Salary (Earning payroll category)
CRA - Codes for T4
https://www.canada.ca/en/revenue-agency/services/tax/businesses/topics/payroll/completing-filing-information-returns/t4-information-employers/t4-slip/other-information-codes-t4-slip.html
T4 – Information for employers.
https://www.canada.ca/en/revenue-agency/services/tax/businesses/topics/payroll/completing-filing-information-returns/t4-information-employers.html
Example of Deduction pay category
Example of Tax pay category
Example of Payroll tax pay category
Example of Benefit pay category
Benefits and allowances explain - CRA link
https://www.canada.ca/en/revenue-agency/services/tax/businesses/topics/payroll/benefits-allowances.html
Benefits chart form assignment
This chart indicates whether you need to deduct Canada Pension Plan (CPP) and employment insurance (EI) from the taxable allowances and benefits discussed in this guide, and shows which codes you should use to report them on the employee's T4 slip.
https://www.canada.ca/en/revenue-agency/services/tax/businesses/topics/payroll/benefits-allowances/benefits-allowances-chart.html
RL-1:
https://www.revenuquebec.ca/en/businesses/rl-slips-and-summaries/sending-rl-slips-and-summaries/sending-rl-slips-and-summaries-to-revenu-quebec/online/
Quebec - Taxable benefits
https://www.revenuquebec.ca/en/businesses/source-deductions-and-employer-contributions/special-cases-source-deductions-and-employer-contributions-in-certain-situations/taxable-benefits/
For additional clarification on form assignment reporting contact your Payroll administrator.