How to classify a region that could qualify as more than one terrain?

[Ultraviolet]

I would just gauge it individually in the different situations.

Type of terrain and vegetation is important, as is climate and season. Also, the activities might affect the skill used. As a GM I'mn open for player-negotiations, if there are valid arguments.

In our apparantly never ending Cliffhangers campaign, on wilderness advantures I often ask for HT based survival rolls to avoid fatigue from travel, weather etc.
If they trek across the African savannah under a blazing sun, Desert might be as good to avoid Fatigue loss, while Plains is used to deal with animals, plants etc.
Same with snow covered, forested mountains. Arctic might be used to avoid frostbite etc. but Mountains is used to avoid falling into cracks etc. and Woodlands perhaps to forage.