A MuPAD® engine is a separate process that runs on your computer in addition to a MATLAB® process. A MuPAD engine starts when you first call a function that needs a symbolic engine, such as syms. Symbolic Math Toolbox™ functions that use the symbolic engine use standard MATLAB syntax, such as y = int(x^2).
Conceptually, each MuPAD notebook has its own symbolic engine, with an associated workspace. You can have any number of MuPAD notebooks open simultaneously.
The engine workspace associated with the MATLAB workspace is generally empty, except for assumptions you make about variables. For details, see Clear Assumptions and Reset the Symbolic Engine.