To define billing series we can go to:

System -> Economic Aux.-> Invoicing Serial number

Then we will see the next screen

Here we have all our billing series defined. 

For example the Series A will be by default for the fees, the series R for the bookings ... Of course we can modify anything to adapt it to our needs. Define our own billing series will make the finance reports very easy, because we will be able to get the reports separated by series if we want to, so we know the amount we get daily, weekly, monthly or yearly for every service we have defined on the billing series.

If we want to edit the series, we can click on

From there we can define our billing series, as well as change their description, set the format of the invoices/receipts for that series, and in Fee series select the service that will be included in this serial in particular.