I had quested previously for the manner in which the 'mtlib' command
-L option is employed, given the lack of info in the device drivers manual
about it.  There was no response from IBMers about it, but eventually I
about it.  There was no response from IBMers about it, but eventually I
figured it out...
    What you code after the -L is a *file name*.  mtlib then takes the
first volser in each line and operates upon it.  This can be useful in
conjunction with the -qI option, to first retrieve a list of the volsers
in the 3494 library and then operate upon it.  One conspicuously valuable
manner of using it is when someone walks up to the 3494 operator station
and invokes the "Reinventory complete system" function, not realizing that
it resets all category codes to Insert, which is not so good.  You can
save the day by doing an mtlib -qI and then use the output to run the
mtlib -C operation to restore the category codes back to your designated
Scratch category, or Private, or whatever (at least for those volumes
you're sure about).
     Richard Sims, Boston University OIT