The number field opens blank and you enter your own. Whip Around does not generate, reserve or check numbers, so the document matches whatever sequence your finance team already uses.
Anything from 1 to 64 characters is accepted, including a prefix of your own such as Q- or INV-.
A number you have used before is accepted without warning, so check it before you confirm.
The PDF is named after the number — Q-1002.pdf, INV-2002.pdf. Characters a computer cannot use in a file name are replaced with underscores in the file name only; the number printed on the document is exactly what you typed.
Because there is no duplicate check, two documents can carry the same number. If several people in your team export documents, agree on who owns the numbering.