The NOCOPY clause tells to PL/SQL engine to pass the variable by reference thus avoiding the cost of copying the variable at the end of the procedure. The PL/SQL engine has requirements that must be met before passing the variable by reference and if those requirements are not met the NOCOPY clause will simply be ignored by the PL/SQL engine.

1) IN parameter mode- This mode is used to pass values to the calling module when invoked. The value of IN parameter can't be changed in the module.

2) OUT parameter mode -This mode is used to return a value to the main block. The value of OUT parameter can change anywhere in the program.

3) IN OUT parameter mode-This mode is used to pass values to the calling module and return a value to the main block. The value of IN OUT parameter can change anywhere in the program.

In Call By value the copy of actual parameter is passed to the formal parameter So any changes to the formal parameter doesn't affect the actual parameter.

In Call By reference the address of actual parameter is passed to the formal parameter so any changes to the formal parameter will change the actual parameter also because both of them are pointing to the same memory location. Here no copying is required.

The IN parameter is passes by reference so we can't change the value of IN parameter inside the module It acts as a constant But the OUT and IN OUT parameters are passed by value we can change the values of OUT & IN OUT parameters if you use the hint NOCOPY with OUT Parameter and IN OUT Parameter then ::: call by reference