IDIQ is the name everyone learns first, but it is one member of a small family the FAR calls indefinite-delivery contracts, all living in the same part of the regulation, Subpart 16.5. The family shares a shape: a base contract is awarded, and actual orders get placed against it later, as needs come up. What differs between them is how strong a promise the agency is actually making you.
Requirements contracts
Under a requirements contract, the agency agrees to buy all of its actual need for that item or service from you during the contract's period, and from no one else. That is a real promise, stronger than anything an IDIQ makes, but it comes with a catch: the agency usually does not know, and is not required to say, how much that need will actually add up to. You get exclusivity, not certainty. If demand for the underlying need dries up, so does the work, and there is no ceiling number to point to, because there was never a shared cap to begin with, only one company's whole requirement.
Definite-quantity contracts
Here the agency commits to a specific, stated quantity, delivered at times it will determine by placing orders across the contract period. You know the total up front, which is more certainty than either an IDIQ or a requirements contract gives you. What you do not know is exactly when each delivery will be called for, so the commitment is firm on amount and loose on timing.
How the promises stack up
| Contract type | What is promised | What is not |
|---|---|---|
| Definite quantity | A fixed total amount | Exactly when orders will come |
| Requirements | All of the agency's actual need, exclusively | How large that need will turn out to be |
| IDIQ | Only the guaranteed minimum | Almost everything above that minimum |
Read top to bottom, the promise gets weaker and the flexibility for the agency gets stronger. Only the IDIQ carries a ceiling, the shared cap the whole vehicle can never exceed. Requirements and definite-quantity contracts do not need one, because there is no shared pool of holders splitting a maximum, just one contract and one company. All three run for a set ordering period, the window during which the agency may place orders at all, and that window is where every dollar of the deal actually happens.
Where each one actually shows up
IDIQs dominate the vehicles built for years of varied work that the government cannot forecast with much precision, whether the seat goes to one company or several. Requirements contracts and definite-quantity contracts tend to show up for narrower, better-understood needs, often supplies or a single well-defined service, bought from one vendor rather than spread across a competed pool. You are less likely to see either one named in a press release, and more likely to just meet the term inside a solicitation's own contract type line, so it is worth reading that line rather than assuming everything indefinite-delivery is an IDIQ.
The naming detail worth knowing
Orders under a supply-focused indefinite-delivery contract are usually called delivery orders. Orders under a services-focused one are usually called task orders. Both work the same way once they are issued: a priced, scoped piece of actual work, carved out of a standing agreement that, by itself, promised you very little.