Personally, I consider "ambient" to be what's measured "in the middle" (with one end being the most added warmth & the other end being unheated). Sorta like saying the "average" temperature, but it's also the air temperature. So if your room/home is chilly, even if you have UTH & overhead heat supplied at one end of the enclosure, if the enclosure isn't insulated, that heat is going to disappear into the room pretty quickly & the ambient temperature in the enclosure on the unheated side may well be too cool for the species of snake you're keeping. Especially for something like a BP that likes tropical warmth- & they won't be happy staying only on the warm side.

Keep in mind that the UTH tends to just heat the floor for "belly heat"- it's not powerful enough to heat all the air, otherwise it would be too hot for the snake to lay on. This is why every enclosure needs a custom set-up, & should be tested for at least a week before a snake moves in, so you aren't going to stress them making needed adjustments. It takes time for UTH to build up warmth when it's properly installed (with air gap below for safety). All heating devices must be regulated, & you need a way to take the temperatures accurately.