15.64.18.25  INTERACTIVE   - This  plots  on  your  terminal.  INTERACTIVE
turns on both PAUSE and FLUSH.  When  this  is  selected  TOPDRAWER  stops
after each plot and waits for you to hit the "Return" key.  If you wish to
stop you type STOP, EXIT, QUIT, HALT, or END.  In addition data is flushed
after  each  line of interactive input.  INTERACTIVE devices support extra
features such as cursor input.   INTERACTIVE  devices  produce  output  on
device  TT:.   If you attempt interactive output to TT:  in a batch queue,
your output will vanish, as TT is assigned to NLA0:.

SEQUENTIAL - Produces plots in an output file

SLAVE  -  Produces  the  plots  at  another  device.   This  is similar to
INTERACTIVE, but the CURSOR is not supported for a slave device.

The default is:
     INTERACTIVE if you are running interactively
     SEQUENTIAL in BATCH mode