Hi Manan,
Houdini is all about “attributes” (on points, verts, prims etc.) one of them is Pscale. Shaders can read attributes and yes in case of particles this is the size of the particle. But you can also use Pscale when copying objects to points (thats the P) or when converting a point cloud to a volume.
You can create any attribute you like and make your own assets that would use those attributes. Pscale is one of the build in attributes and some of the nodes will read it and use it copytopoints is one of them.
@P is another attribute it is the position of that point. Another one would be @N for the normal or @orient for an orientation.
I recommend : my you tube videos (Olaf Finkbeiner) the first one in my series “why i like houdini” is about that.
kind regards
Olaf