http://pwneduufdnxe3dvfwvnzlv5rt4xbfaepc6rw5pajekuwqxe32uivkyyd.onion/FAQs
This allows them to express their email address with an additional piece of data in the alias, usually reflecting the site they've signed up to such as
[email protected] or test+
amazon@example.com. There is presently a UserVoice suggestion requesting support of this pattern in HIBP. However, as explained in that suggestion, usage of plus aliasing is extremely rare, appearing in approximately only 0.03% of addresses loaded into HIBP.