COMMAREA is an overlay with special properties used in the LINKAGE SECTION similar to those other 01 entries you see.
They are probably used as overlays for data retreived by the CICS pgm using the SET clause in such cmds as READ, GETMAIN, etc. See the CICS RefE Manual for othe cmds that use SET.
What I mean by "overlay" is that the datanames in the LS are used to reference the various parts of the data passed or "returned" to your pgm. Remember that the data hasn't "moved" to your pgm; the data address was moved to your pgm and used as a base address for the datanames you defined in your LS.