There are three types of formal parameters in subprograms: IN, OUT, and IN OUT. : Parameters « Function Procedure Packages « Oracle PL/SQL Tutorial

1. Oracle PL/SQL Tutorial
2. Function Procedure Packages
3. Parameters

IN parameters are used to pass values into the subprogram. but can't be changed.

IN Parameters are really a constant.

IN Parameters work in only one direction from the main program to subprogram.

SQL> create or replace function getArea (i_rad NUMBER)
  2  return NUMBER
  3  is
  4  begin
  5     if i_rad is null            -- legal
  6     then
  7       -- i_rad:=10;             -- ILLEGAL
  8       return null;
  9     end if;
 10     return 3.14*(i_rad**2);     -- legal
 11  end;
 12  /

Function created.

SQL>