TL;DRA dated change to a customer's seat balance — plus or minus — that billing reads.
Events are deltas: +5 on the 3rd, −2 on the 18th. The balance after each event is what the invoice sees. Events are stored as UTC calendar days so a date-only sheet never shifts by a timezone.
An add may be pending confirmation for a number of days; a removal in that window cancels pending seats first before it counts as a removal of confirmed seats.
Where it happens in Sonic
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See it on your contracts
A walkthrough on the agreements and files you actually bill from — not a slide deck.