A lot of it depends on where your caliper is mounted. If the caliper is inside the rear triangle, any rack will do. This location is becoming more common.
In any event, that's where my caliper is so I didn't need a special rack and I don't know exactly what makes them special.
Rather than the rack going straight to the dropout, the rack is designed to clear a caliper mounted on the back of the seatstay. Kind of an L-shaped mount, rather than straight. This pushes the rack stays outboard, without which you would have to come up with a DIY installation.