A few questions about structures

I'm new to structures because I never added them before (they where very bugged before 1.8.0) and I have a few questions about them:

1- What's the difference between Surface and Underground spawning in a Nether-like dimension? I tried to make a Lava Well structure generate in my Fire Dimension using "Underground" and it just weirdly generates floating in air or hidden in a place where there is no air (just the one produced by the structure itself);

2- Is it possible to make some structures generate exclusively in a biome that is not an overworld biome? For example, if I want to make 2 different biomes for a dimension, and I want to make different structures based on these biomes, is it possible?

Thanks for helping.