Overview
The Exchange operates as a continuous central limit order book to match orders within the system according to price-time priority:- Higher-priced orders to buy have priority over lower-priced bids,
- Lower-priced offers to sell buy have priority over higher-priced sells,
- Within a price level, older orders have priority over newer orders,
- Should an existing order increase its quantity (at the same price), it is assigned a new timestamp and therefore loses time priority, but quantity decreases retain time priority.
Tick Sizes / Minimum Quantity Increments
Each instrument on the Polymarket US has a minimum price increment (“tick size”) and minimum quantity increment (“lot size”). These instrument-level settings are indicated in the SecurityList [y] message in the MinPriceIncrement (969) and MinTradeVol (562) fields respectively. Order with a price and/or quantity which do not comply with these increments will be rejected using BusinessMessageReject [j] with reason 13 = Incorrect quantity or 18 = Invalid price increment (tick size).Order Types, Time In Force & Execution Instructions
The intended behavior of orders entering the Polymarket US is controlled using a combination of OrderType (40), TimeInForce (59) and ExecInst (18) fields. These are explained in turn below.Table 12: OrdType (40) definitions
Looking for a “classic” Market Order? Simply enter a Market-to-Limit order with IOC Time in Force condition.
Table 13: TimeInForce (59) conditions
In addition to order-level Time In force conditions, participants can also specify a minimum order quantity using MinQty (110), which requires that an order receives at least a minimum execution quantity upon entry (possibly in multiple fills). When used in combination with Time In Force, this field offers fine-grained control over the conditions under which an order can participate. For example:
- An order marked as IOC with MinQty (110) of 200 must receive an immediate, minimum trade quantity of 200. If this is not possible, then the entire order immediately expires.
- In the case of orders marked with a FOK TimeInForce (59), MinQty (110) can be included but has no material effect, as the FOK requires a full fill upon entry.
- A Good Till Cancel order with a MinQty (110) specified must trade at least the specified quantity upon first entry into the order book, otherwise the order expires.