focus ring produces better stacked shots than the same thing taken
on a rail, so use that. Either way can produce good stacked photos.
Using a focusing rail, you mount your camera on the rail, the rail on
your tripod, and by turning little geared wheels incrementally move
your camera closer or farther from your subject, taking photos as
you move along.
Bellows
Lenses can be mounted on a bellows which is then mounted on a
focusing rail for very close macro work, usually in the studio.
Special bellow lenses are often (and usually) used. They are similar
to the old lenses used in enlargers back in the days of film. I am not
going into this here, but some of you may want to learn about them.
Bellows are used mostly for ultra-close macro work. A macro lens
mounted on a bellows, with the front (lens) end locked and moving
that back standard (camera) is a superior way of stacking photos,
superior to using the focus ring or a focus rail.