Booking numbers
Every booking gets a number the customer can quote — 1042, 1043 — and it is unique to your campsite, not shared with anyone else using Campsite Manager. It appears beside the booking and on that customer’s history.
On the Prices page, Booking numbering lets you set an optional prefix (so yours can read HB-1042) and say where the numbering carries on from, which matters if you are arriving from a spreadsheet that already reached 900. You can move that number forwards but not back onto one already used: two bookings sharing a reference is the sort of mistake that only shows up when two groups quote it at the bank.
Gaps are normal. A deleted booking takes its number with it and it is never handed out again. The number identifies a booking; it does not count them. It also never changes once issued — not when the booking moves, not when it is cancelled — because somebody may have written it on a bank transfer.