Are you sure that it's plastic?
It sits exactly where the pistol's extractor would normally be placed. The extractor would necessarily be made of metal.
If it is plastic, maybe it covers the extractor.
Can you remove it? Can you remove it without using tools?
If you can remove it, exactly what do you find underneath it?
The extractor is a spring-loaded, or (sometimes) merely springy, piece of metal with a small hook at its visible end.
The hook grabs onto the new cartridge as it's being fed into the pistol's chamber.
If you ever have to remove the loaded cartridge in the chamber, the extractor's hook pulls the cartridge out when you rack the gun's slide.
When the gun is fired, the extractor helps the weapon's recoil to extract the empty case from the chamber, as the slide moves to the rear. The empty case remains hooked by the extractor until the bottom of the case hits the pistol's ejector (usually a stationary piece of metal). When the empty case hits the extractor, it is thrown out of the extractor's hook, and out of the gun.
On my 96a1, the top of the extractor has red paint on it to indicate a loaded chamber. The red is quite visible when brass or the whole cartridge is chambered.
Love the grips.
On my old (not so good looking) 92FS there is a little niche in the top of the extractor that is painted red. Very similar to the 84 I took apart. there is a pin toward the chamber side of the extractor that holds it and a spring behind the end furthers from the chamber. Mine on the 92 and 84 are metal.