Every invoice has to have a Number, which should be unique. By default, invoice numbers have the format “IV-nnnn” (first invoice – IV-0001, second invoice – IV-0002, third invoice – IV-0003 etc.).
However, you can customize this default format in Settings -> Number Groups -> Edit Invoice Number.
Find out more about Number Groups from here.
This number format will be used for all the next invoices, but if you have assigned specific prefix to your client,
then the invoices created for this client will have numbers with this prefix (for example if the prefix is “CHIP” – then the invoice numbering for this client will start from “CHIP-0001”). When you’ll create a new prefix, the numbering will start with that prefix, followed by “0001”:
Also, you can have a specific prefix on Projects:
If you will generate an invoice based on this project’s tasks (read more here), the invoice number format will include the project-specific prefix, as you can see below:
To wrap up, the Invoice Numbering formats priority is:
1. Project Prefix: if invoice is generated through this project;
2. Client Prefix: if invoice is created for client with specific prefix or if the invoice is generated through the project (assigned to this client) without prefix;
3. Default number: if no prefixes were set for Clients/Projects.