When entering a profile for a UHD-only release there is no possibility to choose a region-code, which is correct, because they are all region-free by standard. When entering a UHD/FHD-combo you can choose a region-code. When entering a UHD/DVD-combo there is not even a region-code field.

Is this region-code meant for the included not-UHD-discs which could be locked to a specific region or should it always be ABC because the UHD-Blu-ray is region-free?

I did it otherwise before, but after thinking again I think the smallest denominator would not be a bad choice here, and we also do it this way with Box-Sets.

Devion27's thread is about box sets with the same media types. I mean combos with an inferior medium included, especially UHD-BD and FHD-BD.

With BD/DVD combos we add the BD region-code (A,B,C) to the main profile. There is no possibility to add the DVD region-code (1,2,3,4,5,6) to the main profile. This means you could have a region ABC Blu-ray and a region 1 locked DVD (most common for Universal Blu-rays). The main profile would say ABC even when the DVD is locked.

If we do it accordingly the main profile for an UHD/FHD-BD-combo always will be region-free, even if the FHD-BD is locked. This might be quite rare nowadays, but Sony and smaller Labels with UHDs like Lionsgate still lock their FHD-BDs of new releases.