FX slot 1 = FX1, FX slot 2 = FX2, etc. If you want, you could have a reverb effect used as FX1, FX2, FX3, and FX4, and have completely different parameters for each one. (In reality, the Qu mixers only have about 8 different effects, but the entries in the library have different parameters that make them sound different) — on a mixer I have, here’s loading the same reverb preset into FX2 and FX4, and then changing the parameters so they are different with the iPad app: https://www.youtube.com/watch?v=NXS2C2S0P-o