In the MIL-STD-490 context, a type of specification containing the technical requirements generically applicable to more than one CI or class of CI, avoiding the repetition of requirements common to more than one detail specification or set of addendum specifications.
These specifications were an early (and effective) implementation of concepts that Object Oriented software engineers later referred to as “inheritance” and “polymorphism”. They permitted a designer to minimize the number of times that a given topic was described for multiple items, avoiding the recurring cognitive processes required to deal with them in many different specifications. I speculate that they fell into disuse because early requirement database designers had trouble allocating a given requirement to more than one target (which would have forced them into an Object Oriented database concept that didn’t exist yet). Or, perhaps, it was just too hard for most inexperienced engineers, and the experienced ones didn’t like to push paper.