From the description of the product, I think the intention is that the tone is received electrically at the end of a cable run, so receipt of the tone indicates continuity of the speaker cabling. This high frequency has been chosen to ensure that it is low enough to be amplified and transmitted along with the audio content, and is high enough to be attenuated by the tweeters (so little energy is transmitted), and high enough in frequency so that whatever is emitted by the tweeters will be inaudible.
If this is the case, it would sound like there is no need to encode any information on the signal, just it's presence is enough to test the wiring is intact. I'm assuming this is so that if the PA is required for, say, an emergency broadcast, it is fit for purpose.
The presence of a <20Khz signal would imply that it was found that in whatever installation was being used that the bandwidth of the amplifiers or some other filter was attenuating the test tone too much to be received properly, so this bodge was put in place.
If this is the case, it would sound like there is no need to encode any information on the signal, just it's presence is enough to test the wiring is intact. I'm assuming this is so that if the PA is required for, say, an emergency broadcast, it is fit for purpose.
The presence of a <20Khz signal would imply that it was found that in whatever installation was being used that the bandwidth of the amplifiers or some other filter was attenuating the test tone too much to be received properly, so this bodge was put in place.