terminal 15 is tech jargon for +12v "swicthed" when the ignition is on.
terminal 31 is tech jargon for constant ground.
terminal 30 is tech jargon for constant +12v (even when the car is switched off).
IF you look at the top of a relay... you will see these numbers.
http://www.lennysvw.com/WIRINGid.htm