The relay has two separate circuits. 30 is the input feed for the load, 87 is the output to the load. Then there's 85 and 86 to the coil in the relay to pull it in to connect 30 and 87. One pin (85 or 86) needs a powered circuit and the other a ground. You can put the switch on the ground side or the power side. Here's a diagram of standard 5 pin relays.
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