There are many different models of Crydom SSR. The ones I use employ an opto-coupler driving back-to back SCRs so they mimic a Triac.
Being for AC only, they turn off at zero crossing of the AC cycle and are turned on again by current in the LED side.
The schematic on crydom site indicates these are SCRs ( **broken link removed** ) but they don't show what is used to turn on or off. The turn-on voltage is 90VAC the turn-off voltage is 10VAC. My understanding of SCRs is they are either on or off, there is no in-between state.
true. There is no "state inbetween". It is just hysteresis to get noise immunity.
You can compare it with a mechanical relay. Maybe it will be safe ON with at 90V, but it will be safe OFF below 10V.
This doesn´t mean the mechanical relay contact´s resistance is somewhere between ON and OFF.