web mapping - The meaning of FeatureCollection in GeoServer WPS

I am new in GeoServer WPS. I am trying to do a buffer around a point table. I use WPS Request Builder to generate the xml request.

I realized that in order to give the input to the process, I usually have three options:

I used text and it was working when I pasted a GeoJSON text in the input box.

When I use buffer functions that have "featurecollection" in their names, I am able to select a layer that already exists in GeoServer Layers and do the buffer on it. In fact, when I select "gs:bufferfeaturecollection" I have 4 input methods (one more option in comparison to "geo:buffer"):

What is the difference between these different buffer methods that are available in GeoServer?:

What does this "featurecollection" mean and why I can only select a GeoServer layer ("VECTOR_LAYER") when I select this method? I mean how can I do other buffer functions on a layer that I stored in GeoServer ( similar to "VECTOR_LAYER")