All of the parameters of the callable services are required, positional parameters. Some programming languages let you pass literal values in these parameters. None of the parameters are optional.

APPC/MVS checks all parameters for valid values, regardless of whether the parameters are used in call processing. For example, the locks parameter on the Prepare_to_Receive service must contain either a 0 or 1 (the only valid values), otherwise APPC/MVS rejects the request to change to receive state with a program_parameter_check return code, regardless of what value is supplied in Prepare_to_receive_type.

See CPI-C Reference for descriptions of the parameters of the CPI Communications services.