Admin users can create custom invoice prefixes for different contracts.
Go to System > Admin  > Types > Contracts  > Add /Edit Contracts popup
The Description  data entry field and the Invoice prefix  data entry field can be alphanumeric with a 25 character/number limit.
The Invoice number length  is limited from 1 to 12, for example:
Invoice number length set to 3.
Invoice start number  range is limited from 1 to 999.
To generate an invoice, please use Quick Ref Create an invoice.