Are the only states of PDLC just ON and OFF?
No, the transmittance (level of transparency) of the PDLC can be varied from 0% in the OFF state up to any value you want until you reach the maximum transmittance (normally 70% or 80%). This is done by simply altering the voltage from 0 VAC up to 70VAC, which is typically the maximum recommended voltage. This can be done electrically with a variable isolating transformer, or electronically with a switched-mode smart glass dimmer.