pass by value
Pass by value is the most natural way to pass arguments to functions.  As the name implies, the value of a variable or expression is passed to the called function.  In the following example, three numeric and two string values are passed by value to Func().

  a = Func (b%, c#+d#, f$$, a$, b$+c$)

pass by reference
Pass by reference is another way to pass arguments to functions.  In other languages, the address of a variable is passed to the called function.  Since the called function accesses the argument through this address, the calling and called function are sharing the variable.  Therefore, if the called function alters its argument, the calling function will find the value of its variable has been changed.

Though functions can return at most one non-argument value, additional values can be returned in pass by reference arguments.  In the following statement, x#,y#,z# are passed to Rotate() by reference so Rotate() can return three values in these variables.

From the values of object,crossSection,vertex, Rotate() computes three DOUBLE values and assigns them to its first three arguments (which might be a#,b#,c# ). Since the calling function passed x#,y#,z# by reference, it receives these final values of a#,b#,c# from Rotate() in x#,y#,z#.

  Rotate (@x#, @y#, @z#, object, crossSection, vertex)

Unlike other languages, arguments passed by reference are actually passed by value to the specified function.  But after the called function returns, the calling function grabs the final value of the argument and assigns it to the original variable.

Therefore the result is the same as conventional pass by reference.  The original variable is altered in accordance with the operation of the called function.  But the native implementation is faster, supports arbitrary mixing of pass by value and pass by reference, and works on variables in registers, which passing by address (ala C) does not.