Which order attributes are mandatory?

The open/close indicator is mandatory because it specifies whether the order is intended to open a new position or to close an existing one. This attribute is crucial for the exchange and market participants to understand the trader's intentions clearly. For example, if a trader wants to open a long position in a futures contract, this must be indicated; otherwise, the order could inadvertently close an existing position instead.

In contrast, while attributes like client ID, limit prices, or algo IDs can provide important context or specificity for trades, they are not universally required. The client ID is typically needed for tracking orders related to specific accounts, and the limit price is generally utilized in limit orders, but not all orders necessitate a limit price. Similarly, the algo ID is only relevant when an order is executed via an algorithmic trading strategy, meaning that it isn't applicable to all trades. Thus, the open/close indicator stands out as the essential attribute that must always be included to ensure clarity in the order's intent.