This section of Part 97 is related to "certification" which is a concept instituted as an attempt to curtail the use of commercially made, amateur (non-FCC-type-accepted) amplifiers by Citizens Band or other unlicensed operators.
Minimum input power and maximum gain specs, modification limitations and no 10 meter operation are technical restrictions that follow this concept.
97.317 says:
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(c) Certification may be denied when denial would prevent the use of these amplifiers in services other than the amateur service. The following features will result in dismissal or denial of an application for certification:
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(5) Any internal RF sensing circuitry or any external switch, the purpose of which is to place the amplifier in the transmit mode;
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This restriction prevents a person from simply connecting a CB radio to an amplifier and being able to "key" the amp. Without these two methods, there needs to be a specific, amplifier-keying (T/R) line available on the radio, which obviously would be missing from CB units. The "external switch" (probably should have been called "externally accessible switch") is, in its simpliest form, a T/R toggle switch on the amp's front panel - while certainly not "automatic", it would easily allow a pure appliance operator's use of an amplifier. RF-sensed T/R would also.
But like typical gov't limitations on technology, this doesn't work very well...amplifier-equipped freebanders just buy amateur transceivers.
Mike