Below is a rough translation description of a rebuild of a renault 1.6L diesel injection pump. I can't read French, if you can,
Add the center spring
Then the disc cover with cams goes over it all.
To assemble this part, the feed pin of the piston (green reference mark) must be aligned with the key.
The cams (bumps) of this disc while passing on the rollers is what moves the piston and compresses the fuel and the injection. The original author had swapped the disc from another model car with a slightly different cam profile.
now, let us move to the piston distributor:
then the drawer of regulation:
This ring, directly actuated by the regulator, will more or less, cover the exhaust hole of the piston (located in the previous photo). During the injection, cam moves, moving the piston. According to the position of the regulator, the hole is released by the depression of the piston. This controls the proportioning of the amount of fuel injected. (when the hole is open, the pressure is released and the injection stops)
The piston is known as the distributor because during rotation, via the holes, it directs the fuel towards one or the other of the exits of the pump.