The way I interpret it is if the player requires a snooker before the shot is taken, then a miss cannot be called.
So for example, let's say a player is 26 points behind and is snookered, with all the colours still on (27 points available). If he plays and misses the shot, a miss can be called as he didn't require a snooker at the time of taking the shot. If he plays again and misses, a miss cannot be called as he is now 30 points behind with only 27 on the table. I may be wrong, but this is how I understand it.
The following is copied from:
https://cuesportkingdom.com/snooker/what-is-the-miss-rule-in-snooker/
//// When can a miss not be called in snooker?
The most common situation (by far) in which a miss is not called, is when one of the players requires 1 or more snookers to win. A miss is never called in this instance.
This is one of the reasons why winning a frame of snooker is incredibly difficult when you need more than 1 snooker. You cannot simply replace the balls if your opponent misses, you must try to get another snooker. ////