What is the purpose of this command? I see it sent by my station at the start of a QSO. I don't see it referenced in the user manual.
Its a new things in V7.1.4. And you mean <TL> which is a shortcut for Test Link.
After a QSY, VarAC will try to send a test to see if the other side is there. If the test does not succeed in 25 seconds, it will assume the QSY failed and will QSY back to the original frequency.
If its not in the manual - I will kindly ask @on2ad.pat who is our manuals master to include it in the upcoming version.