TL;DRWhich mid-period seat changes are charged or credited: none, added seats only, or added and removed.
None bills the period-start balance and charges mid-period adds as a full period, which is what most vendor spillover sheets do. Added seats only charges each add for its remaining window. Added and removed also credits removals.
Under added seats only, the remaining window can be charged at a daily grain or in coarser buckets — half-monthly, monthly, quarterly — without ever being coarser than the product's billing frequency. One invoice sub-line per event, always.
Where it happens in Sonic
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A walkthrough on the agreements and files you actually bill from — not a slide deck.