[Refer to Microsoft documentation.]
MS-DOS shell built-in command called with a filename that invokes the executable associated with the filename's extension. Acts about like double-clicking on a file in File explorer. Usage example from Tcl:
eval exec [auto_execok start] [list $filename]
which, if filename ends e.g. in .htm, calls up IE (if that's your default browser); Win Word for .doc, etc.
[Use in invoking browsers.]
To launch a Windows application (such as Tk) from a console application (such as Tcl) in such a way that it returns only after the subprocess has terminated, use
start /w wish83 myapp.tcl