The difference between a reverse dome and a dish has to do with the shape of the hollowed out area.

A dished piston top has a round depression, usually centered in the center of the piston. This leaves some of the SQUISH area of the cylinder head uncovered.

A reverse dome has the hollowed out area under the open part of the head's combustion chamber. The squish area of piston and head match, almost exactly. It's called a reverse dome or inverted dome because if the piston had a dome it would go up into the open area. The inverted dome has the piston surface go away from the cylinder head over the open area, just like if you took the dome and turned it upside down.

You can argue that an inverted dome is a dished piston and that is technically correct, but the terminology was developed so you have a better understanding of what the shape of the hole in the top of the piston is.

It's easier to say "inverted dome" than to say "it's a dished piston with the dish mirroring the open part of the combustion chamber."

Semantics.

R.