xcall AUI, AUI_IMAGE, IMGOP_SELECTSRC, handle, status {,sourcename}

IMGOP_SELECTSRC allows you to specify which of the available TWAIN data sources (input devices) is to be used for subsequent "Acquire" operations. If sourcename is specified and non-blank, it will attempt to select the corresponding data source from among the available options. Otherwise it will pop up a dialog box allowing the user to select from among the available data sources.

This operation (with no sourcename) should be exactly equivalent to the "Select Source" menu option that appears on the File menu of most programs that support TWAIN.