Hi!
Everthing between -30dBm and -70dBm is good (at least that's how I see it). Performance drops considerably below -80dBm
E.g. if you want AC to perform, you want at least -65dBm.
You will get -30dbm if you are located near an APs antenna. -90dBm means you are close to the range border and you will soon lose connection.
You can configure APs to release clients if the RSSI drops under a certain level, e.g -75dBm. So lazy clients are forced to look for another AP.
Issue is: RSSI says nothing about signal QUALITY, it only measures its STRENGTH.