All of the above...
In most cases with a plastic yoyo the spacer is really the bearing seat. Some manufacturers prefer to make the bearing seat a metal insert rather than mold the into the shell. They call it a spacer for convenience. The FHZ on the other hand comes with both a metal insert bearing seat and a set of washers, which really are spacers used to increase the gap.
A true spacer is just a thin washer between the shell and either the bearing, or the bearing seat if that is a separate piece.
here are examples of true spacers (otherwise known as shims)...




