Foreign Data Wrapper behavior?

In the release notes for postgres 17 I'm reading:

> The PostgreSQL foreign data wrapper (postgres_fdw), used to execute
queries on remote PostgreSQL instances, can now push EXISTS and IN
subqueries to the remote server for more efficient processing.

I'm confused as to what this means. In older versions, are parts of queries
not sent to the foreign server? Or is this change meant to imply the
sending of only the subqueries, the result of which is then directly used
in pushing the entire query?

Or am I still wrong as to what this means?

I looked at the documentation and there doesn't seem to be any indication
of particular queries not being pushed to the foreign server, so this
wording that "can now push EXISTS and IN subqueries to the remote server"
is confusing.

What am I missing?

Regards,
Koen De Groote