ALLOCATE EXTENT 
[( { SIZE integer [ K | M ]
   | DATAFILE 'filename'
   | INSTANCE integer 
   }
   [ SIZE integer [ K | M ]
   | DATAFILE 'filename'
   | INSTANCE integer
   ]... 
 )
]

For information on the symbols and conventions of Backus-Naur Form (BNF) syntax, see Appendix A.