# Time-in-Force & Post-Only

Control what happens to unfilled limit quantity and whether an order may remove liquidity immediately.

Time-in-force decides what happens to unfilled quantity, while post-only prevents taker execution.

## What it does

**Time-in-force** controls whether unfilled quantity waits, cancels, or prevents the order from executing at all. **Post-only** prevents a limit order from taking liquidity.

These are instructions applied to eligible orders, not separate markets.

## How it works

| Instruction | Immediate behavior                          | Unfilled quantity                                                                                       |
| ----------- | ------------------------------------------- | ------------------------------------------------------------------------------------------------------- |
| GTC         | Eligible quantity can fill                  | The remainder stays active until filled, canceled, or ended by another market or order lifecycle action |
| IOC         | Eligible quantity fills immediately         | The remainder is canceled                                                                               |
| FOK         | The complete quantity must fill immediately | The entire order is canceled if a full fill is unavailable                                              |
| Post-only   | A crossing order is rejected                | No taker execution occurs                                                                               |

In the current spot model, post-only applies to an eligible limit GTC order. Market execution is IOC-style rather than a resting time-in-force choice.

## When to use each instruction

| Instruction | Use it when                                                    | Avoid it when                                                         |
| ----------- | -------------------------------------------------------------- | --------------------------------------------------------------------- |
| GTC         | Unfilled quantity may wait for future liquidity                | Any resting remainder would create unwanted exposure                  |
| IOC         | Available quantity should fill now without leaving a remainder | The complete amount must fill or the order should do nothing          |
| FOK         | The complete amount must fill immediately or not at all        | A partial fill would still be useful                                  |
| Post-only   | The order must add liquidity as a maker                        | Immediate execution matters or the price may cross the current market |

A limit price by itself does not guarantee maker status.

Polyester rejects a post-only order that would cross the book. It does not reprice the order to keep it resting, and post-only cannot be combined with a market order.

FOK evaluates all executable matching-engine liquidity within the order's price boundary after self-trade prevention. The visible [Order Book](https://testnet.polyester.com/docs/user-docs/trade/widgets/order-book) can show only part of that liquidity, so its displayed rows are useful context but not the authoritative FOK test.

GTC has no implemented maximum lifetime or hidden expiry timestamp. Pair disablement, delisting, cancellation, or another explicit lifecycle action can still terminate the order, but those events are not a GTC time-to-live.

> **Available combinations depend on the Pair**
>
> The active Pair determines which instructions and combinations are available. Use the controls shown in the current Order Form or supported by the approved API Reference.

## Example

Let's say Bitcoin is currently trading at **100,000 USDT**. You want to place a **0.01 BTC Limit buy** at that price, but only **0.006 BTC** is available immediately. You choose the instruction based on whether a partial fill is useful, whether the remainder can wait, or whether the entire order must fill at once. You choose post-only when your goal is to add liquidity instead of executing immediately.

Here is what each instruction does:

1. **GTC** can fill 0.006 BTC and leave the remaining 0.004 BTC open on the Order Book.
2. **IOC** can fill 0.006 BTC and cancel the remaining 0.004 BTC.
3. **FOK** cancels the entire order because the full 0.01 BTC cannot fill immediately.
4. **Post-only** rejects the order if its 100,000 USDT limit price would immediately match a resting ask.

**Result:** The price and quantity stay the same, but the instruction changes what happens to the order and any unfilled quantity.

*These are example prices and available quantities. FOK evaluates all eligible matching-engine liquidity, which can differ from the rows currently visible in the Order Book.*

## What to watch

- Maker or taker treatment is determined per fill.
- A partially resting order can have both immediate and later fills when post-only is not used.
- Self-trade prevention can end an otherwise eligible match.
- A post-only rejection means the submitted price would have removed liquidity at admission.