The various amp models on the Spider IV give different distortion sounds, and you control how much distortion via the drive setting.
But separate from that, FX1 gives a range of distortion effects options like "screamer" and so on, and you can control the amount of this distortion with FX1's "gain" setting.
But how do these two interact? I don't mean that technically... I don't think... I mean in terms of modeling different tones do people generally use one type of distortion or the other (i.e. amp or FX1), or both together? It seems like most of the factory presets with distortion use both, but that's confusing to me (so many permutations!). If the amp models are meant to mimic various types of amps with their own distortion, why add the FX1 distortion effect on top of that?