The short explanation
Transition Minimized Differential Signaling sends video over three data lanes plus a separate clock, with the clock speed rising as the picture gets more demanding. 4K60 4:4:4 needs a 600MHz clock, which works out at 18Gbps — the ceiling of the TMDS era.
Where you run into it
Most content still runs over TMDS. 4K60 HDR from a streaming box or a Blu-ray player does not need FRL. So when a status readout shows a TMDS clock rather than an FRL rate, that is usually correct rather than a fault.
It becomes a clue when you expected more. If you asked for 4K120 and the link reports TMDS, the two ends never agreed to switch to FRL at all — which usually points at the EDID or at a device in the chain, although a cable that fails FRL link training can cause the same fallback.
One practical note: TMDS termination on a graphics card can interact badly with hot-plug behavior, a known source of odd PC-side symptoms.
How the DR HDMI 8K handles it
The DR HDMI 8K reports the TMDS clock, the clock ratio and the scramble state alongside the FRL figures, so you can tell at a glance which signaling mode the chain is actually using. Firmware 1.2 fixed a hot-plug issue on GPUs when TMDS termination is active.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.