TL;DRCharging per user, licence, or seat, with the count changing over the period.
The base price covers the seat count at the start of the period. Seats added during it are charged from their add date by the product's proration policy; seats removed may or may not credit, by the same policy.
A product can carry a minimum seat count, a per-seat price basis, and tiers. What makes seats hard is not the price but the count: it comes from snapshots, events, or a webhook, and each has to agree on the balance.
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.