BYOC Parametric Multi-Band Compressor Instructions Manual page 14

Step 8: Add the opto-couplers. An opto-coupler is made up of a standard
LED and a LDR (light dependent resistor) joined together in a hermetically
sealed capsule. The leads for the LDR stick out of one end, and the leads for
the LED stick out of the other. Your opto-coupler should have a yellow dot
in one corner. To orient your opto-coupler, match the dot on the opto-
coupler with the dot on the opto-coupler printed on the PCB. You can also
orient your opto-coupler by sticking the longer LED lead in the square
solder pad hole.