If you direct me to context of SRP I can probably work it out ( its not second nature to me yet either!)
PYL is 'pick you label' so upper case would refer to Dom/ Domme/Top/Daddy whoever "pick your label!" And lower case pyl, subs, littles, prey, pets etc.
Edit, I have come to really like 'pyl' as it also means G and I feel more comfortable not picking labels for this, which means we are being us freely, and not worrying about 'stereotypes' or rules. Originally it made me think of sweaters that were past their best and had little balks of wear on them and needed shaving or defuzzing,