You have to be careful and fully read the Infinite Flight ATC Manual, word for word, in order to not encounter issues in interpreting the information. The use of the words should and must can be seen, here’s a reminder as to what both of those words mean.
Should verb – used to indicate obligation, duty, or correctness, typically when criticizing someone’s actions. Used to give suggestions. Example: You should crash, if you can.
Must verb – be obliged to; should (expressing necessity). Example: You must crash, end of story, no questions asked.