The carrier is cantilevered from the ram something like a diving board. If you misadjust a rear die to touch the shell plate, you will put thousands of pounds of pressure on the carrier and will bend or break the casting. The bent casting will distort the hole where the ratchet gear fits rendering the index mechanism non adjustable. In extreme cases the drive bolt will break out through the top of the shellplate carrier casting. The casting must be replaced. The front die (sizing die) can and should be adjusted to stop against the shell plate. By stopping against the shell plate, you will assure the shell plate rises to the same point every time making the most accurate and consistent ammo.
The bullet seating die and powder through expanding dies should not touch the shell plate. Only the sizing die should be make contact with the shell plate, and then only lightly.