When using the Process Next Signature feature to apply multiple signatures to single emails, the 'next applicable' signature will be the next signature processed, that applies to the user and matches the message conditions.
The easiest way to determine this, is to run the Signature Rules Tester. The example below shows a test, simulating a message sent from a specific sender, to an external recipient:
Results will show which signatures are applied to the email. In our example, two policies were applied: Greenorg Standard and Greenorg Reply Signature.