Difference between WRX and FRS/BRZ brake pads
 

I have installed a WRX pad. And it works just fine. But it is missing the spring shown with red arrow. Do you know what is the function of it?

https://i.imgur.com/iZVLWPt.jpg

Replaced brake pads with Hawk Ceramic Performance. I noticed when installing that the front pads were HB432Z.661 instead of HB711Z.661. The one I have was exactly the same dimensions with the other but it was missing that little spring above.
Still worked just fine. So, whatever, I am not returning it. Any comments?

Most aftermarket pads won't have that anyway. It's to try to keep the pad quieter.

Thank you! I don't hear any noise yet. Maybe it is not that necessary.

Yeah, you'll be fine.

The spring clip is there to push the pad away from the surface of the rotor so it doesn't drag when the pedal is not pressed.

The spring clip is way too weak to actually move the pad...

It's actually a part of the pad wear sensor. You're fine if your new pads don't have it.

We don't have pad wear sensors...

It does, however, physically drag on the rotor and make noise, indicating to the driver that the brake pads are low. At least in theory.

Thanks everyone! Well, I sensed my brakes were getting worn-out before they started making noises. I don't need the indicators. The brakes are so much better now that I replaced.

You already have the answer but the technical name is "wear indicator"

When it gets to a lower thickness you get that classic high pitch squeal, which gives you enough time to replace it before rotor damage is done.

If you don't have it and are totally inept (which no one here is) the only squeal would be either an aggressive pad when cold or way later in it's life something rubbing right against the rotor like the rivets on the pad or the like. At that point you've probably damaged the rotor.

If you're smart enough to replace the pads yourself like everyone in this thread is, you probably don't need the wear indicator because you can feel the pads loosing effectiveness and know when to check the pad wear