First of all, you must have created analytical accounts. To do this, see here.
On tab n°4 of the employee management, assign the account(s) by specifying the occupancy rate for each account. The total must be 100%.
If the salary of the employee in question needs to be broken down into more than 10 analytical accounts, the option must be unblocked in the company's settings by ticking Unlimited entry of analytical accounts per employee.
