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

# Orderbook Guide

> How to think about APR, choose a series, and place ST or EPT orders with the right intuition.

Before placing an order, start with two things:

1. pick the **series** you want to trade
2. decide whether you want **principal + yield** exposure through **ST** or **rewards-side** exposure through **EPT**

The same **vToken** can have multiple series open at different maturity dates, and each series has its own **ST/vToken** and **EPT/vToken** orderbooks (though economically they are the same underlying book, as explained below).

***

## What the Orderbook Is

ArcX runs an APR-priced orderbook for two series-specific tokens:

* **ST** trades against **vToken**
* **EPT** trades against **vToken**

You are not placing orders in raw dollar price. You are placing orders in **APR**.

That matters because ST and EPT are both time-dependent claims. As maturity gets closer, the token amounts implied by the same economic deal change. APR is the stable way to express the deal you want.

***

## Why APR Is the Quote Unit

If the book were quoted directly in ST price or EPT price, every order would appear to drift mechanically as time passed.

For example, imagine you want the same annualized ST deal for a series with 60 days left and then again with 10 days left.

* with **60 days** left, that APR implies a larger ST amount per vToken
* with **10 days** left, that same APR implies a smaller ST amount per vToken

If orders were quoted directly in token price, the same economic intention would have to be constantly repriced as maturity approached.

APR avoids that problem.

When you place an APR-based order, you are saying:

* what annualized return you want on the **ST** side, or
* what annualized deal you are willing to accept on the **EPT** side

The token amounts shown in the book can still drift with time, but the APR keeps the economic intent stable.

***

## How to Think About APR by Side

When placing a **limit order**, the cleanest way to think about APR is:

| Order type   | What your APR means                                                                                                        |
| ------------ | -------------------------------------------------------------------------------------------------------------------------- |
| **Buy ST**   | The **minimum APR** you want in exchange for locking your vTokens into the principal + yield side                          |
| **Sell ST**  | You are selling ST at a discount, so the APR is the **maximum discount / worst yield deal** you are willing to accept      |
| **Buy EPT**  | You are paying upfront for rewards-side exposure, so the APR is the **maximum APR** you are willing to give the other side |
| **Sell EPT** | You are selling the rewards side, so the APR is the **minimum APR** the buyer must pay to get equivalent EPT exposure      |

In plain English:

| Side         | Better for you |
| ------------ | -------------- |
| **Buy ST**   | Higher APR     |
| **Sell ST**  | Lower APR      |
| **Buy EPT**  | Lower APR      |
| **Sell EPT** | Higher APR     |

That is the main directional rule to keep in mind when reading the book.

***

## How to Reason About Price

The book is quoted in APR, but users still need a simple way to think about token price:

* **ST** trades below **1 vToken**
* **EPT** trades based on the market's view of future CreditToken generation and reward distribution, and as an EPT buyer you want the price to leave room for your expected return

**As maturity gets closer, the same APR implies different ST and EPT amounts.**

That means:

* a resting order can display different token size later even if the order itself has not changed
* the same APR at different times does **not** imply the same **absolute** return
* percentage return stays comparable, but the remaining notional opportunity shrinks as maturity approaches

***

## Market Orders vs Limit Orders

### Use a market order when:

* you want immediate execution
* enough compatible liquidity is already resting in the book
* the current effective APR is acceptable for your full size
* you are comfortable setting slippage tolerance in the UI

### Use a limit order when:

* liquidity is thin or taking available depth would move your APR too much
* you care about a specific APR more than immediate execution

***

## How Matching Is Checked

ArcX uses an off-chain matching service to find candidate matches between compatible orders. The service proposes fills, but final settlement is checked by on-chain logic.

The contracts verify that each fill stays within:

* the APR bounds the user selected
* the slippage bounds the user selected
* the settlement constraints for the relevant ST, EPT, and vToken transfers

The matcher cannot force a user into a worse APR or an invalid fill outside those on-chain constraints.

***

## ST and EPT Are the Same Orderbook!

**Although ST and EPT may look like two different books in the UI, the reality is that they are the same underlying orderbook.**

The same underlying vToken value can be split into:

$$
1 \text{ vToken} = 1 \text{ ST} + 1 \text{ EPT}
$$

That means pricing either **ST** or **EPT** in vTokens automatically decides the price of the other side.

Or in other words:

* **1 vToken** can always be split into **1 ST + 1 EPT**
* **1 ST + 1 EPT** can always be combined back into **1 vToken**

An order on one side also implies an economically equivalent order on the other side:

* a **Buy ST** is also a **Sell EPT**
* a **Buy EPT** is also a **Sell ST**

For example, a **Buy ST** can match directly with a **Sell ST**. But it can also be satisfied by splitting vToken into **ST + EPT** and selling the **EPT** side. The same logic works in reverse on the EPT side too.

See [Orderbook Mechanics](/deep-dives/orderbook-mechanics) for the full matching logic.

***

## Some More Things to Know About the Orderbook

* Limit orders can be partially filled
* Cancelling an order only unlocks the unmatched remainder
* Market orders let you set slippage tolerance in the UI so the final result does not drift too far from the quoted APR
* You cannot match against your own resting orders

***

## Next Steps

* Read the [User Guide](/learn/user-guide) for the step-by-step trading flow
* Read [EPT Pricing](/deep-dives/ept-pricing) for rewards-side return intuition
* Read [Orderbook Mechanics](/deep-dives/orderbook-mechanics) for the full math