The T-bill indexes and the Average Prime Offer Rate index (APOR) are two separate indexes on loans secured by a consumer's principal dwelling. Treasury securities are used to determine if a loan is "high-cost" under §32 of Reg. Z. The APOR is used to determine if a loan is "High Priced" under §35 of Reg. Z, and HELOCs are exempt. Both loan definitions have two spread tests, one for first lien and one for second lien.
Several states have high cost mortgage laws that are similar or identical to §32 of Reg. Z in definition.