This joint is known as a “step lap rafter tail,” and the joint in the plate is known as a step lap rafter seat.
I believe it gets its name as the tail laps over the plate, which creates an overhang to shed the rainwater away from the side of the building. And that the notch cut into the seat looks like a stairstep.
In this drawing, the rafter is a 5×5, and the tail that laps over the plate is 1 1/2″ thick, which is the same dimension as the step. Usually, they are both the same.
The rafter has been cut back with an arc to help eliminate a “point of fracture” where the roof load could split the timber.
The plate in this drawing is a 6×6.
This shot is of the end of the plate and the gable rafter.