What is the distinction between the terms ‘shallʼ and ʻshouldʼ in a user requirements document, which is written in natural language?

What is the distinction between the terms ‘shallʼ and ʻshouldʼ in a user requirements document, which is written in natural language?

Ans: Mandatory requirements are requirements that the system must support and are usually written using 'shall'. Desirable requirements are not essential and are written using 'should'.