I'm afraid, MadFred, that you're partly wrong. (A rare thing, readers.) You are totally right about the description part and making the text appear. However, the show, never show etc part is to do with whether the waypoint will show as black marks on the map. It is nothing to do with yellow boxes.
If you want to see yellow boxes play in Cadet mode. You can't change thereto in the Editor, so if you're not getting your yellow box fix go back to the main menu, select single missions and choose Cadet. You don't actually have to start a mission (I think). Go back to the Editor and it will be yellow box city. Providing there are waypoints there for you to see of course.
Remember that it is very poor mission design to make things happen at player waypoints. Lots of people only play in Veteran mode and the mission must work for them too. Waypoints should be just a Brucie bonus for Cadets.
Oh, and actionmancarl
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