i wonder what is staging mode in the asm pz..

Look at it this way. If all of your attack signatures are enforced and set to block and you have the policy in blocking mode the attack signature will be blocked. If F5 releases a signature update or you add in a different signature, that particular signature will be in staging until you enforce it. This is a precautionary measure to insure that there will be no blocked traffic after updating signatures. Once you feel confident that you like the change made you can move it from staging into enforced to block on that signature.

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