When does Postgres use binary I/O?

Hello,

I've read the docs at [1] and also this interesting recent post about
adding binary I/O to the hashtypes extension. I wrote send & recv
functions for my new multirange types, but I'm not sure how to test
them. After running installcheck or installcheck-world, the code
coverage report says they aren't tested, nor are the send/recv
functions for ranges or arrays. When does Postgres actually use these
functions? Is there a good way to test them?

Thanks,
Paul

[1] https://www.postgresql.org/docs/11/xtypes.html
[2] http://www.myrkraverk.com/blog/2019/08/postgresql-retroactively-adding-binary-i-o-to-a-type/