What does this message about Fallback Licensing mean? I don't understand!

You have purchased Chinese/Taiwanese hardware that doesn't comply with SMBIOS standards that the rest of the world uses (we're not being racist or ideologically bigoted - until such time as we find any evidence to the contrary, this is an objective fact).  Your hardware fails to provide the expected genuinely-unique hardware identifier (Universally Unique IDentifier, or UUID), and consequently ThinLinX regards your hardware provider as "untrustworthy", and has fallen back to an "if all else fails" form of licensing.

Tms_client has detected - by means of a hardcoded "known offenders" list - that you are using a mainboard claiming a known-to-be-non-unique UUID.  Consequently, ThinLinX has no recourse but to use your device's primary MAC address (see below for the definition of "primary") as your unique hardware identifier.  This has significant limitations:

The licensed MAC address will be that of the network interface that corresponds to the device's primary IP address, as shown in TMS or Tlxconfig, and if that primary interface changes after the device is licensed, the MAC address will no longer match, and your device will revert to a 30-day trial license.

In other words, if you substantially alter the networking arrangement of a device using fallback licensing, e.g. change from Ethernet to WiFi or add a VPN, then you will invalidate your license.

FYI the "primary" network interface is defined to be the interface corresponding to the IPv4 default route with the lowest metric, if there is a default route, or the non-loopback IPv4 route with the lowest metric otherwise.

Your primary network interface may change during the boot process, and consequently fallback licensing may not work properly on offline networks, because it will require access to our Internet license server to update the license when network interface changes occur.  You may see a bogus warning
about your device being unlicensed that is only temporarily true, and you may also see a duplicate entry in TMS because of the serial number change.