Basically what this plugin does for you is add the ability to have replaceable tags in your signatures that are context sensitive to the Mail account that you are sending from. In addition, you can have it autofill information from iTunes into a signature as well. Each account can also have an entire replaceable piece that is used for that account only if a certain tag is in the signature. That's how the "Professional Assasin" appears above. It is what is called the "Tail Signature" for the Phil Anthro account.