# Settlement - How trades become final

When you and your counterparty are matched off-chain, the sequencer creates an `IntentFillRecord` for each side of the match. It then packages all the fills from one match into a **batch**, identified by `batch_id`.

The batch is submitted on-chain via `BatchSettler.batchSettle(IntentFillRecord[] records)`. The contract iterates through every record and calls `Market.settleTrade` on the appropriate market contract. Each `settleTrade`:

1. Verifies the signed intent (as described above).
2. Verifies the margin: `fillSize ≤ MarginCalculator.maxSize(collateral, leverage, fillPrice)`.
3. Determines what kind of action this fill represents based on the trader's current position:
   * **Open** if they had no position.
   * **Increase** if same side as their position.
   * **Reduce** if opposite side, smaller than their position.
   * **Close** if opposite side, equal to their position.
4. Locks or releases collateral on the Hub accordingly.
5. Updates the position record.
6. Emits an event (`PositionOpened`, `PositionIncreased`, `PositionReduced`, `PositionClosed`).

#### Atomicity

The whole batch is atomic. If `settleTrade` for *any* fill in the batch reverts (invalid signature, exceeded `priceLimit`, intent expired, anything) **every fill in the batch reverts**.

This is critical. It means you can never end up with a one-sided settlement: if Alice opens a long, somebody must have opened the matching short, both recorded in the same transaction.

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