>Line 118 of /sys/sys/ttycom.h defines UIOCCMD(n) to be _IO('u',n) or
>a command op code (an ioctl which doesn't transfer data in or out).

This was intended as a special sort of covert channel on ptys --
you can do a UIOC on either side (master or slave) and then pull
out the value of `n' later on the master side.  It shows up as
an exception in select and a special byte on a controller read
(see ptcread()), as long as PF_UCNTL is set.

Beyond this much, I have no idea what it was/is for.

Chris