This is how Tally knows who a person is. Staff don't get a separate Tally password — they sign in through Stratum Suite, and Tally matches them to their account by this email. It needs to be the same address they use for Suite.
If a user can't get in, the most common cause is an email here that doesn't match the one on their Suite account. They must be the same.
Technical section
Data model, API, and internals — visible to developers.