1Z UPP (no fillers or silicones) = SFX1
1Z PP (silicones present) = SFX2
1Z MP (silicones present) = SFX3
Both 1Z MP and SFX3 have very limited polishing capabilities and are meant to finish perfecting a near perfect finish. Most of the time you can go straight from 1Z PP to LSP, but I always like to use a cleaner step after PP, which I usually go with AIO or VM but if you prefer to stick with the 1Z line and aren't topping with a sealant, you can use MP. 1Z designed their GW to go right on top of their polishes so MP is a perfect intermediate from PP to GW. I must say that MP is one of my least used products, but only because I like to always use sealants.
If you're looking for a pure polish without oils/silicones, I'd go with 3M PI III RC and PI III MG. The RC isn't quite as aggressive as 1Z UPP but it does a great job. MG is = PP, but no oils.