signature is equivalent to the XCALL parameter of the same name, and is used only if the XCALL parameter is blank or not present. The theory behind putting it here is that you may wish to include the same “signature” text on every message, yet every installation will surely have their own (typically listing important contact information for the company), so why bother having to reference it explicitly on every email. Note that there is nothing magical about the “signature” – it is just text (possibly “styled text”) which could just as easily be included with the body parameter or not at all.