The purpose of a software requirements specification (SRS) is to specify external behavior of the product. It should be specific enough that all stakeholders reach the same conclusion about the product's intended behavior. A different document created by a designer will specify a software architecture and/or algorithms.
If requirements writers (product managers) find it difficult to specify external behavior unambiguously without resorting to design, then a disclaimer should be included:
Warning: The "design" contained herein is supplied as an aid in understanding the product's external behavior only. The designer(s) may select any design they wish provided it behaves externally in a manner identical to the external behavior of the above system.This is a good reminder to both the product manager that they are not the designer or architect, and to the designer that selection of optimal architectures and algorithms is their responsibility.
Reference:
Davis, A., Software Requirements: Objects, Functions, and States, Prentice Hall, 1993.
