If it has two small metal pins along the long edge, the small rectangular piece of plastic is for opening the battery cover. Otherwise, I have no idea.

The cadence magnet as it comes in the box is actually a two piece unit, allowing you to adjust for height (or distance to the sending unit). You can remove the base (it just snaps on) if the magnet on your crank arm is too close to the sending unit when they're all in the right places. I needed to do that, since the sender had to be just slightly ahead of my rear wheel on the chain stay, and this placed the magnet very close without the base.

You have to experiment with the positions of the magnet and sending unit, since the magnet can only be a max. of 12mm away from the sender. It's not as sensitive as the speed unit, apparently.

Oh, and you HAVE to use the zip-tie with the cadence magnet, or it'll get knocked off for sure.

FWIW, I did end up using the zipties on the cadence sending unit once I found the best spot for it. Just made itmore secure andless likely to get banged off.