Invoice numbers
Short answer
An invoice gets its number when you finalise it, such as INV-0001, and each one is the next in your sequence. A number is never skipped or used twice, even if the invoice is voided.How numbers are given
A draft has no number. Pressing Finalize gives it the next one in your sequence: INV-0001, then INV-0002, and so on. After INV-9999 comes INV-10000.
The number is given as part of finalising, so two invoices finalised at once still get two consecutive numbers, and one that fails to finalise gives its number back.
Never skipped, never reused
A voided invoice keeps its number and stays on record, so your numbers never have a gap that needs explaining. A discarded draft never had one.
Test mode has its own
Invoices in Test mode are numbered separately from your real ones, so practising does not use up your numbers.