> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mercurjs.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Order commission lines

> Per-order commission lines, how they're computed, and BigNumber precision.

This page covers how commission is recorded on an order and kept in sync as the
order changes.

## Commission line

A commission line is the resolved commission for a single order line. It's
represented by the `CommissionLine` data model (table `commission_line`, id
prefix `comline`). Each line anchors to either an item (`item_id`) or a shipping
method (`shipping_method_id`). It records the rate that matched
(`commission_rate_id`, `code`), the applied `rate`, and the computed `amount`.

```ts theme={null}
// A commission line, as written by the refresh workflow
{
  item_id: "ordli_123",
  shipping_method_id: null,
  commission_rate_id: "comrate_123",
  code: "standard-a1b2c3",
  rate: 10,
  amount: 250, // 10% of a 2500 subtotal
  description: null,
}
```

Lines are generated automatically during checkout. When the cart is split into
per-seller orders, `refreshOrderCommissionLinesWorkflow` runs against each new
order. Shipping lines carry a `"Shipping Commission"` description. Item lines
carry none.

## Recomputed on change

Commission lines are **derived** data, not a one-time snapshot. The lines are
recomputed whenever an order's composition changes, such as when an order edit
is confirmed, a return is received, or a claim or exchange is created. That
keeps the seller's commission tracking what the customer actually kept.

The refresh is **idempotent**: it deletes any existing lines for the affected
items and shipping methods, then inserts the freshly computed set. Re-running it
never duplicates lines.

<Note>
  Computed lines carry no `id`. The module's `upsertCommissionLines` deletes by
  anchor (`item_id` / `shipping_method_id`) before inserting, which is what
  makes repeated refreshes safe.
</Note>

## BigNumber precision

All commission arithmetic uses Medusa's `MathBN` (BigNumber). It's arbitrary
precision, so percentages and per-currency amounts never accumulate
floating-point error. A percentage amount is `subtotal × value ÷ 100`. A fixed
amount is the per-currency value (or the fallback `value`). Each is computed in
BigNumber before being stored.

<Tip>
  Commission lines feed the payout pipeline: when a seller's payout is
  calculated, the order's commission lines are read straight from the commission
  module and deducted from the amount transferred to the seller.
</Tip>