What does “in writing” mean?

in the form of a letter, document, etc. (that gives proof of sth)

How is “in writing” used?

In Paths of Glory (1957), one line is: “unless it is in writing and signed by the general. Over.”

The clip at the top of this page is from Paths of Glory (1957).

unless it is in writing and signed by the general. Over.

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