What are parameter types in SAP ABAP?

There are two types of parameters in SAP ABAP.

The first type is Formal Parameters. It is defined during the definition of subroutine with ‘FORM’ keyword. The second type is Actual Parameters. It is defined during the subroutine call with ‘PERFORM’ keyword. 

There is another way of defining parameter types as input and output. Input parameters are used to pass data to the subroutine while output parameters used to pass data from the subroutine.