What does REL stand for?

Runway Edge Lights

Are provided for a runway intended for use at night or for a precision approach runway intended for use by day or night. Runway edge lights shall be fixed lights showing variable white, except that: in the case of a displaced threshold, the lights between the beginning of the runway and the displaced threshold shall show red in the approach direction; and a section of the lights 600m or one-third of the runway length, whichever is the less, at the remote end of the runway from the end at which the take-off run is started, may show yellow.