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The modal verb ‘must’ can be used to express both obligation and deduction.
obligation – being told to do something and you have no choice but to do it
You must have a driving licence to legally drive a car in England.
To see a GP, you must register to be a patient at their practice.
You need a driving licence to drive a car; if you drive without a licence, you are breaking the law.
You need to register with a GP surgery if you want to see a doctor. If you are not registered at their surgery, the doctor will not see you.
deduction – a process of thinking in a logical way
You must be happy to have passed your driving test.
The medicine must be working.
In these examples we use the modal verb must to indicate deduction.
We think/expect that you are happy because you have passed your driving test. It is logical to be happy in this situation.
We can see that the sick person who has been taking medicine is looking better so we think/expect that they are improving because the medicine is working.