In the case of "big procurement" one typically aims for a requirement/technology capability target, some 3, 4 or even 10 years beyond conceptual design phase.
Suppliers will have to base their design on a projected level of technical development which will not initially be understood, and to complicate matters, the original business case may subsequently change beyond all expectations to accommodate new threats, requirements, etc.
In my experience, Clients generally tie down the window of acceptability to a very small area consequently it becomes very difficult to pass.
COTS rapidly becomes unworkable for anything except the smallest projects.
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