System Spool Directories
------------------------

`SYSTEM'
     There is a subdirectory of the main spool directory for each remote
     system.

`SYSTEM/C.'
     This directory stores files describing file transfer commands to
     be sent to the SYSTEM.  Each file name starts with `C.G', where G
     is the job grade.  Each file contains one or more commands.  For
     details of the commands, see See UUCP Protocol Commands.

`SYSTEM/D.'
     This directory stores data files.  Files with names like
     `D.GSSSS', where G is the grade and SSSS is a sequence number, are
     waiting to be transferred to the SYSTEM, as directed by the files
     in the `SYSTEM/C.' directory.  Files with other names, typically
     `D.SYSTEMGSSSS', have been received from SYSTEM and are waiting to
     be processed by an execution file in the `SYSTEM/X.' directory.

`SYSTEM/D.X'
     This directory stores data files which will become execution files
     on the remote system.  In current practice, this directory rarely
     exists, because most simple executions, including typical uses of
     `rmail' and `rnews', send an `E' command rather than an execution
     file (see The E Command.).

`SYSTEM/X.'
     This directory stores execution files which have been received from
     SYSTEM.  This directory normally exists, even though the
     corresponding `D.X' directory does not, because `uucico' will
     create an execution file on the fly when it receives an `E'
     command.

`SYSTEM/SEQF'
     This file holds the sequence number of the last job sent to
     SYSTEM.  The sequence number is used to ensure that file names are
     unique in the remote system spool directory.  The file is four
     bytes long.  Sequence numbers are composed of digits and the upper
     case letters.