Samsung SCH-R640 User Manual page 219

A sum of 5 is considered acceptable for normal use. A sum of 6 is
considered for best use.
T3 + T2 = 5
M3 + M2 = 5
In the above example, if a hearing aid meets the M2 level rating and
the wireless mobile device meets the M3 level rating, the sum of the
two values equal M5. This is synonymous for T ratings. This should
provide the hearing aid user with "normal usage" while using their
hearing aid with the particular wireless mobile device.
"Normal usage" in this context is defined as a signal quality that is
acceptable for normal operation. The M mark is intended to be
synonymous with the U mark. The T mark is intended to be
synonymous with the UT mark. The M and T marks are
recommended by the Alliance for Telecommunications Industries
Solutions (ATIS). The U and UT marks are referenced in Section 20.19
of the FCC Rules.
The HAC rating and measurement procedure are described in the
American National Standards Institute (ANSI) C63.19 standard.
Health and Safety Information 214