Yes. It’s the “window” or boundaries of that which is thought of to be acceptable public discourse.
For instance, there may be a range of differing “middle-ground” views on a particular topic which nevertheless are deemed acceptable.
For example, in the abortion instance, most European countries permit abortion on demand (or virtually on demand, labelled “social reasons”) until a point that varies between 12 and 24 weeks, depending on the jurisdiction, with abortion on medical grounds permitted later.
Holding a view that abortion on demand should be permissible until 12, 16, 18, 20 or 24 weeks would be considered “mainstream” and within the window, but a view that abortion should be banned completely even if a woman’s life is at risk, or at the other extreme that abortion should be permitted up to the moment of birth would be considered extreme and outwith the window (or on the fringes of it).
It surprises me that the “do not engage” people on this topic are probably some of those who criticise the “do not engage” approach of TRA’s.
Ironically, it is often those who are on the extremes themselves that like to think that “their” position is right and should should not be up for debate.
I would like to think that my own views on most things are actually quite mainstream, but I think it’s much healthier to debate with extremists than just shut them down.