Reason for 'should' over 'must':

1. Recommendation:
Jessica is advised to revise her notes before the exam. The word 'should' implies a suggestion or recommendation, rather than a requirement or obligation.

2. Flexibility:
Using 'should' gives Jessica the flexibility to decide whether or not to revise her notes. It suggests that it would be beneficial for her to do so but does not impose a strict mandate.

3. Courtesy:
By using 'should' instead of 'must', the sentence conveys a polite and considerate tone. It shows that the speaker is offering advice rather than giving a command.

4. Possibility of alternative actions:
If 'must' were used instead of 'should', it would imply that there is no choice for Jessica but to revise her notes. However, by using 'should', it leaves room for other study methods or strategies that Jessica may find more effective.

5. Encouragement:
The word 'should' can also be seen as an encouragement for Jessica to take the initiative to review her notes, rather than feeling pressured or obligated to do so.
In conclusion, the use of 'should' in the sentence "Jessica should revise her notes before the exam" is appropriate as it conveys a gentle recommendation, allows for flexibility, maintains a polite tone, and encourages Jessica to take responsibility for her study habits.