The MCI command tables for some device types, including MCI_DEVTYPE_VCR and MCI_DEVTYPE_DIGITAL_VIDEO types, provide support for the MCI_TEST flag. The driver receives the MCI_TEST flag in the lParam1 argument to DriverProc.
If an application includes this flag with a command, the driver does not initiate the specified operation. Instead, it determines if the operation is currently available. If the operation is available, the driver returns zero. Otherwise it returns MCIERR_NONAPPLICABLE_FUNCTION. For example, a VCR driver might not allow an MCI_INDEX command while a seek operation is in progress. (Of course, in such a case the driver should return MCIERR_NONAPPLICABLE_FUNCTION even if the MCI_TEST flag is not specified.)