When you save an invoice, Sage Sole Trader gives it the next number in your sequence. The number goes up by one each time you create an invoice.
Once Sage assigns a number to an invoice, that number stays with it. You can't reuse the number, even if you later void the invoice.
When you void an invoice, Sage keeps the number assigned to the voided record. Your next invoice takes the next number in the sequence. This makes the voided number look missing, when in fact it's still in use. The sequence continues from the next free number.
No. You can't reuse a number once Sage assigns it. This keeps a complete audit trail of every invoice you create, including any you void.