A virtual wall does not allow the parts to interfere, but allows them to separate. This video covers it pretty well: Advanced Fixtures in SolidWorks Simulation - YouTube
It's like the "no penetration" contact in nonlinear analysis.
Contact like this makes any problem nonlinear. It's my understanding that the 'virtual wall' is just a tool that allows the more complex behavior to occur in a SW "static" analysis. The solver is using nonlinear analysis of some kinds behind the scenes.
The advanced fixture, of course, restrains both interference and separation by fixing movement in the specified direction.
A Virtual Wall definition behaves not unlike a No Penetration contact with a flat part. A model is not allowed to penetrate the wall but is allowed to lift off. With an On Flat Face fixture the displacement in the normal direction is specified and no movement in either direction (either penetrating the wall or lifting off) will be able to occur.
Ryan & Keith,
Thank you for your input. You both mentioned that the flat face advanced fixture in the normal direction restricts interference and separation from the specified face. Why does that constraint offer the "Reverse Direction option?". When I first noticed this, I always thought you restrict movement in one direction, not both.
The On Flat Face fixture is allowed to have a non-zero value entered. When that happens this 'Reverse direction' toggle is what dictates which side of the face the prescribed displacement moves toward. That option doesn't have meaning with a 0 displacement entered.