There are two different kinds of contracts. The first type is a “Time and Material” contract, which tracks the quantity of items ordered over a given timeframe, and is generally used for recurring item (product and/or service) purchases from a supplier. The items placed on a purchase order from a “Time and Material” contract must be determined at the time of creating the contract.
The second type is a “General” contract, which tracks the amount paid out of a monetary total during a given timeframe, and can be used for tracking things like subscriptions to services over time. The items placed on a purchase order from a “General” contract are not locked in when creating the contract, allowing any database items and/or external items to be ordered and tracked through the contract.