The primary reserve is the first line back up for the regular force. In other words, the P Res train on a regular part-time basis, conduct their own exercises etc. They are primarily used to augment the Regular Force in manpower shortfalls for CF operational committments. 56 per cent of all P Res Captains are on some form of extended full-time contract in support of the regular force. DND forks over 8.2 million dollars a year in salary to reservists for augmentation to train the regular force. That's outside of augmentation money used for overseas committments.
The supplementary reserve (from my understanding) is an organization of prior Regular Force or Primary Reserve members who exist only on paper as a means to track qualified or trained personnel should there be a need for a call-up. You can transfer from the supplementary reserve to the Primary Reserve or even to the Regular Force without having to under go basic training again etc...(could be wrong on that one but I'm working on the memory system here).
In the past, supp reserve guys used to put on their uniform for one day a year and parade together and get paid for it. That practice has been abolished.
When you get out of the Regular Force or Primary Reserve, you can opt to be released completely from the forces or transfer to the supplementary reserve. Hope this helps.
PJ D-Dog