In Caterbook, every invoice has an internal ID that relates to the booking, in the format I-XXXX-N.
I- means this item is an invoice
XXXX is the booking reference number
-N is the numbered invoice for this booking.
I-1132-1 would be the first invoice for booking 1132.
I-1132-2 would be the second invoice for booking 1132 - perhaps where a business traveller is paying for their own food and drink but the first invoice is for accommodation.
This numbering system is purely for internal use and to make it quick and easy to understand what booking an invoice relates to.
Legally, invoice numbers must follow on sequentially and so there is also a public facing / accounting invoice number that appears on each invoice generated. In the above example it might be that the first invoice (I-1132-1) was created when the booking was made for a corporate partner on 1st April and has sequence ID 1064. The second invoice (I-1132-2) is created on checkout on 25th June and has sequence ID 1097.