Control Data Corporation 3300 Reference Manual page 34

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The files on an output multifile reel are written in the order dictated by the
logic of the program rather than the order in which they are defined in the
MULTIPLE FILE clause. If an output multifile reel is to be used as input
to the same program, the input-output control system assumes that the files
were written in the order specified by the MULTIPLE FILE clause.
The input-output control system assumes that all files on the multifile reel
have the same type of labeling.
Since no two files on a multifile reel may be open simultaneously, they share
buffer and record areas automatically. No file which shares a reel with
another file can extend across reels; that is, a file cannot be on a multifile
reel and also be multireel.
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