The parking brake is like the old drum brakes. They are smallist shoes which contact the inside of the rear disc.
There is also a click adjuster like the old drum brakes.
Best thing is to slacken off the handbrake cable, have the discs removed, sand-paper the shoes and the inside of the rear disc where the shoes contact, then re-assemble and adjust the click adjuster so that the shoes do not bind. Then adjust the cable so that it takes 3 or 4 clicks to engage.
As with the old style drum brakes, the contact surface of the shoes occasionally gets clogged with brake dust and need to be "cleaned" to get them working effectively again. If you ever get your rear discs replaced I suggest you have this done to your handbrake shoes at the same time while everything is apart.