Omnisphere is a multi-timbral instrument, allowing up to eight different Parts to be played at the same time within a single instance of the plug-in.
Each of the eight Parts can be thought of as an entire synthesizer, complete with its own oscillators, modulation routings, and FX, and each Part can be played on a different MIDI channel.
The settings of all eight Parts are saved with a MULTI, or with your project. Everything will be recalled by loading that specific MULTI or project. The settings of a Part can also be individually saved as a Patch.
A Patch contains all the settings that make up a sound, and can be individually loaded into any of the eight Parts.