What is the CR product of the photorelay?

The CR product is one of the important indicators for a photorelay to handle a high-frequency signal through the switch between the output terminals. "C" in this CR product refers to the output capacitance when the photorelay is OFF, C_OFF. "R" is the resistance between the output terminals when the photorelay is ON (ON-state resistance), R_ON.
When C_OFF is high, even if a photorelay is turned OFF, a high-frequency signal will pass through. If R_ON is high, loss when a signal passes through and deterioration will occur. Therefore, for applications that switch high-frequency signals, a photorelay with small C_OFF and small R_ON, i.e. a photorelay with low CR product, is required.