Urgent Operational Requirement.
You can have it now, and for the next two years, but there is no plan to keep it it service any longer than that, which is why there are no spares. Or part numbers for spares. Or supporting publications. Or specialist qualification courses for the REME to allow them to fix the UOR stuff. And if it is still serviceable we will sell it off anyway.
It does get kit through to the user a damm site quicker than the usual procurement paths though: and if this isn't damning with faint praise, I don't now what is.