# Order Types and Matching

### Order Types and Matching

The platform operates on a **limit-order basis**. Clients can **buy options to open positions** and **sell options to close positions**. All orders are **GTC (Good Till Canceled)**.

#### Order Types

**BBO (Best Bid Offer) Order**

* A limit order placed at the current **best available price**.
* If the market price changes while the order is being processed, the order is placed at the requested price as a limit order.
* Orders execute **for the available volume**. Any unfilled contracts remain in the order book as a limit order.

*Example – BTC Call (BBO Buy)*

* Instrument: `BTC-250926-115000-C`
* Current Order Book: **Bid 425 / Ask 428**

1. **Scenario A – No price change**
   * User places a **BBO buy order for 10 contracts at 428**.
   * Ask remains **428** with only **7 contracts available**.
   * Result: **7 contracts execute immediately at 428**, and the remaining **3 contracts rest in the order book as a limit order at 428**.
2. **Scenario B – Price change**
   * User places a **BBO buy order at 428**.
   * Ask moves to **431** during processing.
   * Result: A **limit order at 428** is placed in the order book for the full amount (pending fill).
3. **Scenario C – Price change**
   1. User places a **BBO buy order fro 10 cpntracts  at 428**.
   2. Ask moves to **420** during processing and 20 contracts available. 
   3. Result: 10 contracts are executed immediatley at 420.

**Close Order**

* Using the **Close button** submits a **sell BBO order** for the open position.
* If the market drops while processing, the order is placed at the requested price as a limit order.
* Orders execute **for the available volume**; any remaining contracts rest in the book.

*Example – BTC Call (Close)*

* Open Position: Long 0.05 contracts `BTC-250926-115000-C`
* Current Order Book: **Bid 422 / Ask 426**

1. **Scenario A – No price change**
   * User clicks **Close**.
   * Bid remains at **422**, with full size available.
   * Result: **All 0.05 contracts are sold immediately at 422**.
2. **Scenario B – Partial fill**
   * User clicks **Close** to sell 10 contracts at **Bid 422**.
   * Only **6 contracts are available** at 422.
   * Result: **6 contracts sell immediately at 422**, and the remaining **4 contracts rest in the order book at 422**.
3. **Scenario C – Price change**
   * User clicks **Close**.
   * Bid falls to **419** during processing.
   * Result: A **sell limit order at 422** is placed in the order book (pending fill).

**Limit Order**

* Placed directly in the order book at the **specific price** requested by the user.
* Remains active until matched or canceled.
* Supports **partial fills** — executed volume reduces the open amount, with the remainder staying in the book.

**Sell Limit Order (for Open Positions)**

* Lets users set a **target exit price** for their open position.
* Partial fills are possible if liquidity is less than the order size.

#### Matching Engine

* **Centralized Order Book**: All orders are placed in the book and matched by the engine.
* **Execution Rule**: Price-time priority (best price first, then earliest order).
* **Partial Fills**: Supported. Any unfilled contracts remain GTC in the book.
* **Order Book Visibility**: At (Phase 1), the order book is not displayed to clients, though all orders still rest and match in the book.


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