Structure Selection from Set

So, I just had a quick question for clarifying how structure sets work. 
Here's how I think they work: Within the spacing square, a random point is chosen and a random structure from the structure set is chosen. If the chosen structure cannot generate at the chosen point (i.e. it's the wrong biome), the structure will not generate.
However, the thought crossed my mind that maybe the chosen structure is guaranteed to work with the chosen point. Like if the random point is chosen first, then the list of generatable structures is "reduced" (in quotes because maybe the set will just pick another structure if the chosen one is not valid for the point); or if the structure is chosen first, the possible points at which it can generate is also "reduced".
So, my question is which is it? The first method basically shows that there is no guarantee that a structure will generate in the spacing square, whereas the other method does guarantee a structure to be generated within the spacing square.