Field help (AADRVAL)

The mode defines how the parameter is transmitted in the call. It is impossible to modify it.

By address

A reference is sent to the call, and any modification made in the call on the variable will directly change the value of the parameter.

By value 

A parameter is copied and the value sent can be modified during the call with no impact on the parameter value.

Constant

A reference is sent to the call, but the parameter is read only. Any attempt to modify it during call execution will generate an error.