Explanation of HAC Ratings.
Prior to 2024, each device with rating from the FCC had a separate M-rating and a T-rating (see below). From roughly 2024 on, this has essentially been replaced with a single rating of "hearing aid-compatible" that is equivalent to M3 T3. This is based on the 2019 ANSI standard and is sometimes displayed as "HAC-2019" or similar.
"M" ratings refer to general compatibility / interference levels between a device and a hearing aid. "T" ratings refer specifically to a hearing device's telecoil ("T Switch" or "Telephone Switch").
With both M and T ratings, a higher number is better. M3 T3 is considered good but M4 T4 is the best rating.