what is meant by ByVal and ByRef?

When using byval, you pass the argument's value. When using byref, you pass the argument by reference. If you pass by value, the variable can not be changed by the calling structure. If you use byref, the variable can be changed by the calling structure.

Is there ever an exception to that defination, that when using ByVal you pass only the value and the variable can not be chnaged by the calling structure?