"A" is where the cheek meat is, your collar may or may not come with it since if the cheek is prized by itself fishmongers may remove it to sell separately.
The collar is where "C" is and contains fatty meat and delicious goodness. Cheek may or may not be included depending on how it's cut.
I see. My parents usually buy the entire salmon and cut it themselves. I will look for it next time. Thanks!