They are set on a site by site basis.
Unfortunately I don't have the in-depth technical knowledge to explain exactly what is happening, other than it has something to do with levels needed to cause the scanner to react to the incoming signal.
There is no "universal" number. Each radio system and user's location has certain variables affecting the signal. So we are all in the "trial and error" mode when it comes to determining which values work best for the given system we want to monitor.
See the screen shot below on where these adjustments are found in Sentinel, when editing a Favorites List: