Final answer:
In a user requirements document, 'shall' indicates a mandatory requirement, while 'should' suggests a recommendation.
Step-by-step explanation:
'Shall' in a user requirements document indicates a mandatory requirement, meaning that it is a necessary condition that must be met. On the other hand, 'should' suggests a recommendation, meaning that it is not mandatory but recommended to be implemented. These terms are commonly used in natural language to provide clarity and specificity in conveying requirements and recommendations.