The short explanation
Fixed Rate Link is the newer way HDMI moves data. Where the older TMDS method scaled its clock with the picture, FRL runs at fixed steps and packs the video into them. It uses up to four lanes instead of three and runs each lane faster, which is where the extra headroom comes from.
The steps run FRL1 through FRL6: roughly 9, 18, 24, 32, 40 and 48Gbps. A status line reading FRL 5 / 10Gbps – 4 lanes / 40Gbps means four lanes at 10Gbps each.
Where you run into it
The FRL rate is the single most useful number in an HDMI status readout. It is the negotiated result, not a claim. If your chain reports FRL5 when you expected FRL6, you know where to look first — the maximum FRL rate advertised in the EDID, or a link that could not train at the higher rate, not HDCP.
A cable that cannot sustain the higher rate is the usual reason. So is a device that only implements the lower rates despite an HDMI 2.1 badge.
How the DR HDMI 8K handles it
The DR HDMI 8K displays the live FRL rate, the lane count and the per-lane bandwidth on both the OLED and the web interface. In the EDID flags you can also cap the advertised FRL rate deliberately, which is the cleanest way to test whether a marginal cable is behind an intermittent fault.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.