Expander result

It says

by first expanding them into a minimal dialect of @rnrs{7} (small)
without any syntactic extensions.  The resulting expression or program
is then evaluated by an existing Scheme implementation.

From what I can tell, the scheme implementation that does evaluation after expansion doesn't need to be fully r7rs conformant, only the implementation that bootstraps the expander. What specifically can be omitted from the runtime's Scheme implementation, and still have it run successfully?