How to write System requirements - not all architectures fulfill all requirements

Question

We have a system that is largely configurable, that can be organized in different architectures, and I'm struggling to write its requirement specification. I'll give an example

• Module 1 does A
• Module 2 does B
• Module 3 does C

The system can be configured to have any combination or all of the three modules. How do I write the requirement?

1) The system shall be configurable by Engineering to do A.

2) The system shall be configurable by Engineering to do B.

3) The system shall be configurable by Engineering to do C.

4) The system shall be able to do A.

5) The system shall be able to do B.

6) The system shall be able to do C.

I like the first 3. The next 3 however will not necessarily always be true (if system is configured with modules A and B, requirement 6 will be false).

Shall I complete requirements 4 to 6 with "if the system is configured to do so"? Are 4 to 6 necesssary at all?

Thanks for your help!

EDIT: So basically the question is, how do you write a requirement when the system should be able to something only under a certain configuration?

Answer 1

1. Functional requirements

1.1. System shall be able to do A, B, C.

2. Non-functional requirements

2.1. Configuration management

2.1.1. System shall be configurable to do one or more of the features mentioned in FR#1.1 simultaneously.

2.1.2. It shall be possible to change configuration mentioned in NFR#2.1.1 on the go / with service interruption for up to one hour / etc.

Answer 2

How about a truth table

        configuration
          1  2  3
feature A t  f  f
        B t  t  t
        C f  f  t