Most new messages require additional parameters to specify their exact function. The media control interface messages use pParam2 as a pointer to a data structure, and use ulParam1 as a bit-field for the flags associated with the message. A flag will exist for each field in the data structure that accepts data from the calling application. The application uses the flag to set the bit-field of ulParam1 to indicate a value is assigned to a particular field. Flags also specify options without parameters. Thus these flags do not correspond to a field in the data structure.