No offence but PPR is passport request letter, I think passport transmission letter means the email that consists of awb two way courier service for all those who have booked the service.(That’s what I feel)
This is required by VFS not by IRCC
IRCC just needs PPR, Passport and photos and even if transmission letter means PPR adding 1 extra document will cause no harm.
I sent my awb email printout also along with PPR just for the safer side.
I would say when in doubt always send more/extra documents.