This should explain it
Guys the picture above is for a starter relay (intermittent duty) not continuous duty contactor (ie master or Bat relay).
Master (Bat) Contactor: Besides the two BIG terminals which is common on all contactor's (relays), battery in & out, you have either one or two small terminals for the coil. If there's one small terminal, it's wired to the master switch to ground. A "catch diode" goes from the single small terminal to the large POS terminal on the contactor, battery side. The diode strip goes towards to large POS terminal. See three terminal pic below. The pos side of the coil is internally wired.
With the Four terminal version, the second small terminal gets and external "jumper" to the positive BIG terminal on the battery side of the relay with a wire (diode not needed). The diode now goes between the two small terminals, with the stripe end towards the positive side of the coil (the terminal with the jumper - see pic below). The difference between the one/two small terminal versions is the 4 terminal version has an external jumper to the pos side of the coil, the 3-terminal version is internally jumped.
Three post Master Contactor (continuous) CLICK
Four post Master Contactor (continuous) CLICK
Starter Contactor: Is similar in terminals to the master, in that they come in three or four post versions (one/two small terminals), but when you have the forth post, you just don't use it. There is no extra jumper. The small terminal labeled "S" is for starter switch. It is always used, and activation is with a POSITIVE momentary fused switch. The other side of the coil is grounded. The small 4th post you don't use is labeled "I" for ignition. On some cars this goes to the Ignition coil. In previous text some one says its for starter engaged light. It could be used for that. If you want a starter engaged light and don't have that 4th "I" terminal, you can go off the big terminal on the starter side. The 4th small terminal (if installed) is just internally tied together with the big output terminal when the relay is closed/activated. The "catch diode" is wired as shown above, from the "S" terminal to grounded, usually mount bracket bolt.
Here's a great gouge from our friends at Vertical Power that explains relays with pictures. By the way, ignore PAGE 5. That's probably your relay but you don't need all those diodes shown. Just one diode as shown above will work great. Picture on page 5 is for a cross-tie application, which justifies all the diodes I was told.
http://www.verticalpower.com/docs/Contactor_Wiring.pdf
Side notes: Typically the master is all metal (silver finish) and starter is a dark composite plastic housing. Note in the one picture above, the master is mounted pointy/cap end up and the starter metal cap (terminals) down. (This avoids inadvertent activation from pos vertical G-force overcoming spring.) With the starter relay B&C sells, it does not matter (it mounted horizontally). However personally the horizontal mount starter relay B&C sells does not fit as nice as the ones Van's sells (see pic above). B&C's solenoids are not superior in quality or function to the ones Van sells; they just have different configurations. There all stout, simple, heavy duty industrial relays.