Field help (ATSSAL) 

Specify in this grid the salaries corresponding to the 12 calendar months preceding the employee’s last day worked and paid.

For example:

Last day of work:25/05/2001.
Specify the salaries from the 01/05/2000 to the 30/04/2001.

This information is used in the Pôle Emploi (French Job center) certificate and the UI-19 form (South-African certificate).