Version added: before 9.5
Return type: boolTrue if the layer itself is selected, no matter how many layers are selected or if this is the working (active) layer or not.
String "SecondarySelection" is found in the following scripts:
Layer Finder (by Lukas)
MR Tween Machine (by eugenebabich)
SS - Cycle Keys (by simplsam)
SS - Reference Layer Update+ (by simplsam)
MR Transform Rig Tool (by eugenebabich)