There is an Enhancement Request for Department, Class and Location to have Auto-Generated Prefix Number –
Enhancement 146453: Setup > Company > Auto-Generated Numbers > Transactions; Be able to have a check box for Department, Class and Location.
As an alternative solution you may add the Numbers manually before the name.
1.) Navigate to Setup > Company > Departments
2.) Click Edit on the department to be edited
3.) Add the number before the name of the department
4.) Click Save.
For example:
100 Marketing
200 Accounting
300 Logistics
This will affect the Transactions created and to be created.