# Market accounts

When an Econia user places a limit order or a market order (or if a custodian places one on their behalf), the user's assets and orders are tracked in a MarketAccount that is specific to the given market and custodian.

As explained in the user module documentation, the concatenated result of market and custodian IDs is known as a "market account ID", which is used for MarketAccount lookup inside a user's MarketAccounts as well as inside of a Collateral resource for any relevant coin types.

## Collateral​

A user's MarketAccount tracks the amount of base and quote assets held, as well as the amount available for withdrawal, but it does not actually store any coins held as collateral. Instead, a separate Collateral resource is maintained for each relevant coin asset. This includes:

Coin Collateral is stored separately from a user's MarketAccount to reduce type arguments for assorted functions, and is routed directly between counterparties during a trade. More specifically, when a user deposits assets to Econia for trading, their assets are held locally rather than in a global vault for the entire order book. Through this direct peer-to-peer trading approach, Econia:

• Eliminates transaction collisions that would otherwise result from deposits and withdrawals against a global resource.
• Reduces the size of a hypothetical economic attack target (a central vault) by distributing its contents across all users.

## Asset counts​

An Econia MarketAccount tracks a user's open asks and bids, as well as the following fields:

FieldMeaning
Base totalTotal base asset holdings
Base availableBase asset holdings available for withdrawal
Base ceilingAmount base total increases to if all open bids fill
Quote totalTotal quote asset holdings
Quote availableQuote asset holdings available for withdrawal
Quote ceilingAmount quote total increases to if all open asks fill

For example, consider the following sequence:

1. A user opens a MarketAccount for an APT/USDC market.
2. The user deposits 100.00 USDC.
3. The user places a bid for 5.00 APT at a price of 4.00 USDC per APT (20.00 USDC total).
4. The bid is completely filled.
FieldBefore depositAfter depositAfter bid placedAfter bid fills
Base total0.000.000.005.00
Base available0.000.000.005.00
Base ceiling0.000.005.005.00
Quote total0.00100.00100.0080.00
Quote available0.00100.0080.0080.00
Quote ceiling0.00100.00100.0080.00
tip

Decimal units are presented here for the sake of illustration, but Econia uses integer units internally.

## Collateralization​

Econia requires complete collateralization within a given MarketAccount: base available and quote available amounts must always be nonnegative. This means that every position on an Econia order book is fully backed with an asset that cannot be withdrawn unless the corresponding maker cancels their order.

Note, however, that Econia still supports margin trading through its issuance of custodian capabilities. For example, a third party can operate its own internal borrow/lend pool and then trade borrowed assets on a user's behalf as a delegated custodian, cancelling orders and withdrawing assets to cover lenders in the case of a liquidation event. Here, there is no risk that an Econia order is unbacked (even in the case of borrowed assets), because a trade cannot be placed until the actual underlying asset is deposited into Econia and marked unavailable for withdrawal. Instead, the risk of default is assumed by lenders external to Econia.

In addition to taker orders placed via a market order from a MarketAccount, Econia also supports swaps that do not require an underlying MarketAccount or even an aptos_framework::coin::CoinStore: the amount of collateral required to cover the trade must simply be passed as an argument to the relevant function.

## Generic assets​

In the case of a GenericAsset for the base asset on a market, an underwriter is required to certify base asset amounts, either within a MarketAccount or during a swap.