Within the subroutine, a formal parameter can be used in the same way as a local variable. An actual parameter, or argument , of a subroutine is an expression, a variable, a function, or a constant that gets passed to the subroutine at runtime in place of the corresponding formal parameter. Parameters that appear in the parameter line of a subroutine are value , variable , and constant parameters. Value parameters are not preceded by the words var or const in the parameter line and are passed into but not out of a subroutine.
Variable parameters allow information to be passed into and back out of a subroutine.
Constant parameters cannot be changed within the subroutine.