ByRef: The argument is passed by reference. ByRef is the default.
ByVal: The argument is passed by value. Its value can be modified by the called routine.
char: Type declaration character.
typename: Primitive data type name. Library or module defined types can also be specified.
= expression: Specify a default value for the argument, matching its declared type. Optional is necessary for each argument specifying a default value.
ParamArray: Use ParamArray when the number of parameters is undetermined. A typical scenario is that of a Calc user-defined function. Using ParamArray should be limited to the last argument of a routine.
UsingParamArray or = expression require Option Compatible to be placed before the executable program code in a module.
When using Option VBASupport 1, Optional arguments with no default value (= expression) are initialized according to their data type, except if Variant.