Please correct me if I'm wrong, as per my understanding:
"Devoir" has multiple meanings, usually depending on the context; it can either be (literal or figurative) debt, obligation, or probability. It's also usually translated to 'must' or 'to have to'.
With that being said, consider the following sentence:
L'avion doit atterrir à dix heures.
How can one tell if it can be translated as:
The plane is supposed to land at ten o'clock. (probability)
The plane must land at ten o'clock. (literal translation)