Constraint

In technology selection, a constraint is a condition that limits which solutions are permissible and that is imposed from outside the design trade-off: a law or policy, an approved data location, an existing contract, a budget, a deadline, the staff available, or a mandated platform. NASA’s systems-engineering guidance describes constraints as conditions the design must adhere to, or that limit how the system will be used, and notes that they typically cannot be changed through trade-off analysis.

Constraints rule options out before scoring

“Invoice data must remain in the approved region,” “no new operating platform before the current contract ends,” and “two people are available to support the system” are constraints for a lookup project. A product that stores data elsewhere is excluded before its features are compared. A constraint therefore works like a mandatory condition, and it should be justified in the same way: name the source and the person who owns it.

Without a named source, a preference can pose as a constraint. “We must use the vendor we already know” is a preference unless a contract or supported-environment rule actually says so. Ask where the sentence comes from, and write the answer beside it.

Only the owner can relax it

A constraint is often written in the same form as any requirement; the difference is who may change it. A scoring formula cannot lift a data-location rule, and a low price cannot compensate for breaking it. If a constraint blocks every workable option, return to its owner for a decision and record what was relaxed and why.

Constraints also expire or change: a contract ends, a budget is revised, a platform is retired. Date them and note when they should be rechecked. NASA separately describes design constraints that arise from decisions made as the design evolves; keep those apart from stakeholder constraints so that a choice made by the project is not mistaken for a rule imposed on it.

Reference: NASA: Technical requirements definition. Examples here are illustrative.