What are the contact-a and contact-b of the photorelay?

The contact-a turns on the output side when a current higher than the fixed value is running to the input side. This contact method is also called a normally open type.

The contact-b turns off the output side when a current higher than the fixed value is running to the input side, and turns on the output side when the current on the input side drops lower than the fixed value. This contact method is also called a normally close type.