When is it ok to use symbol "#" vs. the word "number" in writing? Ex. Vendor # or Vendor number; Job # or Job Number?
It is generally unacceptable in UK English, except where there is not room even to write no. or where this could cause confusion. Of course nothing is stationary and its use is increasing, although some consider this an Americanism. I will let others answer for other varieties of English but please make it clear which variety your answer relates to.