The Invoice Number and Invoice Name are two distinct things. An Invoice Name is an internal reference used by you to identify the invoice at a glance. Whereas, the Invoice Number is displayed in the invoice and visible by customers. See below.
Invoice Name
By default, AppColl names invoices based on the invoice date and customer. Invoice names can be manually changed when the invoice is created.

Invoice Number
AppColl automatically increments invoice numbers each time a new one is created. By default, invoice numbers begin with 000001 and increment to 000002, 000003, etc. Any time an invoice's number is manually changed, AppColl will increment with the new format.

For example, assume that AppColl has generated invoices 000001 - 000006, and the next invoice is manually changed to CUST-001. AppColl will increment the next invoice as CUST-002, CUST-003, etc. This can be changed whenever a new invoice is created if you wish to maintain custom invoice numbering for each client. However, you would need to remember the last customer-specific invoice number that was used. We generally recommend leaving the invoice numbering untouched to avoid gaps in numbers. 

An invoice can have more than one Invoice Number. More information on multiple invoice numbers can be found here.

Please note that invoice numbers are actually text strings, thus allowing you to add alphanumeric characters. For example, 1, 01, 001 and 000001 are all considered unique values.