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

# FIFO Order Book

> Price-time priority and user-facing orderbook behavior

Phoenix uses a FIFO order book for normal resting orders.

FIFO means:

* better price fills first
* at the same price, older resting liquidity fills before newer resting liquidity

## What a resting order stores

A resting FIFO order stores:

* the maker trader position
* initial and remaining base size
* order flags
* optional conditional order index
* optional slot-based expiry
* initial slot

## Capacity and order limits

Phoenix enforces both per-trader open-order limits and market-level orderbook capacity.

* A trader can have up to `64` resting bid limit orders per market.
* A trader can have up to `64` resting ask limit orders per market.
* The per-trader limit is per market and per side, so one trader can have up to `128` resting FIFO orders on one market: `64` bids and `64` asks.
* IOC and other take-only orders do not count toward this cap because they do not rest on the book.

## Order packet families

At the protocol layer, the core order packet kinds are:

| Packet kind         | User-facing mental model                | Rests on book                    |
| ------------------- | --------------------------------------- | -------------------------------- |
| `PostOnly`          | maker-only limit order                  | Yes                              |
| `Limit`             | standard limit order                    | Yes, after any immediate matches |
| `ImmediateOrCancel` | IOC, market, or FOK depending on fields | No                               |

## Important fields

### `price_in_ticks`

The order price in native book ticks.

### `num_base_lots`

The total base size to fill or rest.

### `last_valid_slot`

Optional expiry slot for the order.

### `match_limit`

Optional cap on how many resting orders an incoming order can match against.

### `cancel_existing`

Advanced behavior that lets some order paths cancel conflicting resting orders instead of failing a margin check immediately.

## Maker vs taker behavior

* A resting order that provides liquidity is maker flow.
* An incoming aggressive order that removes resting liquidity is taker flow.
* A standard limit order can act as either, depending on whether it crosses the book.

## Post-only behavior

Post-only orders are for adding liquidity only.

* If they would immediately trade, they do not execute as takers.
* Depending on configuration, they may slide to a valid resting price rather than cross.

## Expiry and live L2 views

FIFO order expiry is slot-based. An expired order can remain in the raw book until a matching or maintenance path touches it. Consumers that need a live L2 view should filter expired FIFO orders before aggregating by price.