Features
Quotes and credit notes
What comes before and after the invoice. Numbered for you, with nothing quietly deleted.
The quote
Made just like an invoice, with its lines, VAT and withholding, but it doesn't count towards your taxes. Send it by email or link, and mark it accepted or rejected.
From quote to invoice
When the client says yes, “Convert to invoice” creates the invoice with the same lines, its number and its date. No retyping.
The credit note
An issued invoice isn't deleted or quietly changed: it's corrected. The credit note goes in its own series, R, with negative amounts and a reference to the invoice it corrects, with its number and date. And it subtracts in the quarter it belongs to.
Numbering
Consecutive with no gaps, a series per issuer and whatever pattern you like (the year and a counter, say). You don't keep count of anything.