What is 4K120?

In plain language

Twice the frames of 4K60, so roughly twice the data. At 4:4:4 10-bit it needs FRL and comfortably more than 32Gbps. A 4:2:0 variant exists for chains that cannot carry the full version.

Why it matters in your setup

A console or graphics card will only offer 4K120 if the capability data it reads says 120Hz is available. The chain has four chances to lose that: a display that only supports it on certain inputs, an AV receiver that passes video but advertises its own lower ceiling, a cable that cannot sustain the FRL rate, or an EDID that got truncated on the way.

The symptom is not an error message. The mode simply does not appear in the source's list, and users conclude their hardware does not support it.

What the DR HDMI 8K does with it

The DR HDMI 8K includes twelve 4K120 EDID tables — 4:4:4 FRL-5 10-bit and 4:2:0, each with and without VRR and HDR and in 7.1, 5.1 or 2.0 audio. Present one and the source is told 4K120 exists, full stop. If the mode then appears but the picture drops out, you have learned something specific: the capability was the missing piece but the link cannot sustain it, and the cable or the FRL rate is next.