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, COFF. "R" is the resistance between the output terminals when the photorelay is ON (ON-state resistance), RON.
When COFF is high, even if a photorelay is turned OFF, a high-frequency signal will pass through. If RON is high, loss when a signal passes through and deterioration will occur. Therefore, for applications that switch high-frequency signals, a photorelay with small COFF and small RON, i.e. a photorelay with low CR product, is required.