Complex to Real Projection is an option available if modes from either File 1 or File 2 are real and modes coming from the other file are complex. In this situation, complex modes will be converted to real modes with the following methodology:
Find the maximum component for complex mode shapes and calculate its phase.
Rotate all the components of the complex modes by the phase calculated in the previous step, so that the maximum component is now a pure real component.
Keep the real parts of the rotated components.
The MAC Calculation Method will then use Real to Real formulation.