You are slightly confused by the terms. "Plain" refers to the crankshaft bearing -- a "plain" bearing is also called a journal bearing, or a bushing --- instead of a set of ball-race bearings.

A ringed engine has a piston ring to seal the gap between the loose-fitting piston & the wall of the cylinder. This is the same type of arrangement that is used in automobiles & just about every other reciprocating internal combustion engine, although the larger engines use more that one ring. When the ring wears out, the engine can usually be overhauled just by fitting a new ring & some minor prep-work to the cyl walls.

Many model engines used lapped pistons instead of piston rings. In this configuration there is a very close fit between the un-ringed piston & the cylinder -- close enough to seal against compression & combustion pressures without using a ring. Years ago, the pistons were hand lapped into the cylinder with a very fine abrasive, to make the tight fit. This fit is specific to each engine & when the fit wears too much the engine needs a new piston & cyl sleeve.

Now with CNC machining, I'm not sure if they are lapped anymore.

"Lapped" piston types include ABC, ABN, steel-mehanite, aluminum-ceramic -- & I'm sure that I'm forgetting some other combinations here.