When passing arguments to a procedure (sub or function) the default will be ‘byRef’ in Excel VBA, thus you don’t have to write ‘byRef’ before the variable. If you want to pass ByVal you have to explicitly write ‘ByVal’ before the variable. When using ByVal you are only passing a copy of the argument to the function or sub and the original value is not changed. When passing arguments you always want to declare the variable as this will help your code run faster. If you leave out the declaration. Excel will assume as variant which takes longer to process. About 2.5 times longer according to the tests below.
Coding with Visual Basic 