EOS 650 M button function?

[Russell Boate]

Could someone advise me as to the purpose of the buttom with the M on it
located just bellow the DOF button on the front near the lens mount.  I
purchased the camera without a manual and am waiting to receive the one I
bought on Ebay.  I have noticed that in Manual Mode when i hold the M button
and rotate the wheel the lens stops open and close, what I dont understand
is the viewfinder display which changes from OP to oo to CL
Any help would be appreciated.

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