Difference between IN, OUT, IN OUT

IN	OUT	IN OUT
It is the default.	It must be specified.	It must be specified.
It passes values to a subprogram.	It returns value to the caller.	It passes initial values to a subprogram and returns updated values to the caller.
It is a formal parameter acts like a constant.	It is a formal parameter acts like an uninitialized variable.	It is a formal parameter acts like an uninitialized variable
It is a formal parameter cannot be assigned  a value	It is a formal parameter cannot be used in an expression and must be assigned a value.	It is a formal parameter should be assigned a value.
It can be a constant, initialized variable, literal or expression	It must be a variable.	It must be a variable.