Thanks @jazzyisj!
After some searching and reading … list all sensors with state unavailable none or unknown as well as @w35l3y comment in Difference between unknown available none… (my googling was bad as I did not find a source of truth in the HA docs 
) this seems to be the reasoning what to choose:
Rule #1: all states are strings. So you must not think of none being 0. It’s just the defined state whatever happened behind the scene.
- unavailable in case of an error or as @w35l3y says if it does not have deterministic state. E.g. you don’t know if a binary sensor is true or false (on or off) although everything seemes fine with the setup and sensor.
- unknown in case the sensor was not reached or setup yet. E.g. after booting homeassistant when the state was not set yet it’s often unknown
- none could mean unknown or some other (hopefully) specified state of a sensor
So all the availability and template availability checks are to determine the deterministic state while just setting the state to unknown you flag it as “Right now I don’t know pal”
Would you agree?