Checkout-driven scheduling is rule 4 of Lemon's sync and the reason its booking identity is anchored on the checkout date (rule 1). Forward-trims move the check-in daily; merges move the check-in; a guest arriving a day late moves the check-in. None of those change when the flat needs to be cleaned. In the 30 days to 2026-09-10 Lemon logged 67 check-in-only changes that did nothing to any job, against 29 checkout changes that rescheduled one.
The principle also decides what a cleaner is told. A check-in change produces no message, because nothing about the cleaner's day changed; a checkout change produces one message with the old and new time. Late checkouts are the one exception in the same direction: the checkout stays on the same day but the start time moves, so the cleaner receives the new time and is asked to acknowledge it.