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Level 4

On the properties tab of the signature field you have and option to lock fields after signing.

You can either lock all, lock none or choose a collection of fields to lock or not to lock.

Its pretty flexible.

If you want your form to have multiple signatures and be used in reader 9 you'll have to use this property no prevent the locking of the other signature fields otherwise you won't be able to use multiple signatures (even if you certify the form first).