https://en.wikipedia.org/wiki/New_Zealand_Football_Championship#Clubs There's no documentation of this that I've found yet. I'll search for some when I have time. Not sure what "slot protection" is. Thought maybe it meant a relegated club would be replaced by a club of that same general area, thus preserving that geographical "slot" Anyone from down there know what's going on? Also, I thought the regional league systems had seasons opposite to the NZFC. Is this incorrect? Is it being changed? This is all intriguing, though my understanding is pro/rel every 4 years, which almost seems pointless.
I'm guessing that "slot protection" means that there are more clubs than protected slots, so each area is guaranteed a club and could have more. The areas that have more can be changed by promotion and relegation. The analogy would be to a Champions League where each country gets at least one club, the best have more than one, and coefficients change how many clubs each country gets. I'm not saying that's how New Zealand will do it, but that's my guess.
And actually, so is the whole NZFC. It is being replaced by the new NZNL from '21 forward. https://en.wikipedia.org/wiki/New_Zealand_National_League