Here’s another piece of jargon. How many of you have heard something described as a “red line issue” as in “We regard the removal of Trident as a red line issue.”?
Like “pushing the envelope”, it’s connected to flying. Aircraft have a maximum safe speed they can travel at, indicated by a red line. You’ll see a similar line on the rev counter of your car. So a “red line issue” is an issue people won’t back down over, something they cannot cross.
Unfortunately, there are a lot of other possible interpretations. If something is underlined in red, it’s being emphasised, so a “red line issue” could be one that’s important.
Similarly, in Microsoft Word’s Track Changes feature, edits are made in red and underlined, so a “red line issue” could be something that needs to change.
And there’s the historical “Thin Red Line” I allude to in the title, which could lead your reader to assume that a “red line issue” is something defended by a small number of people.
There’s even the practice of “redlining” in the United States, where jobs, loans, mortgages and healthcare are restricted to certain parts of a city. Districts outlined in red on maps, usually inhabited by racial minorities, would be denied these services. So a “redline” issue is not the same as a “red line” issue.
I suggest avoiding all “red line” issues, as they’re almost certain to confuse your reader.