Tier Description?

I understand that Delta uses a "Tier" system in ranking potential candidates. Does anyone know what makes up a Tier? I've read that Delta is currently interviewing Tier 1 candidates; what are the credentials that make up Tier 1?

Also has anyone who had an interview scheduled that was canceled when hiring was suspended last time been called for an interview? I had a interview scheduled and I received an email telling me it was canceled. Anyone else in the same situation?