When you create an invoice, the system will give it the number 001 by default and increment this number for the next invoices.
But you can change it manually:
if you want the numbers to begin at a certain point (e.g. you already created 20 invoices with another tool before using LSP.expert, so you want the first LSP.expert invoice to be No. 21)
if you want to add the year (e.g. 2017-035)
if you want to add your name or something else in the invoice number
To change the invoice number manually, you have to click on Actions > Edit invoice details when you're in an invoice.
Please note that the system will do its best to increment the numbers correctly. If you define 2017-0035, it will of course name the next invoice 2017-0036. But if you defined 0035-2017, for example, it may increment it to 0035-2018, so make sure the number to be incremented it at the end.
Also, if you change an invoice number to add the name of the client in it (e.g. Babel Translations - 2017-0035), please note that the system will keep the name of this client in the next invoices you create, even if they concern other clients (because it only increments the number).