The Free On-line Dictionary of Computing (30 December 2018):

file control block

    (FCB) An MS-DOS data structure that
   stores information about an open file.  The number of FCBs is
   configured in CONFIG.SYS with a command

   	FCBS=x,y

   where x (between 1 and 255 inclusive, default 4) specifies the
   number of file control blocks to allocate and therefore the
   number of files that MS-DOS can have open at one time.  y (not
   needed from DOS 5.0 onward) specifies the number of files to
   be closed automatically if all x are in use.

   (1995-03-21)