I have a test–use pointwrangle change a grid like @scale=rand(@P) or @pscale=rand(@P),then put a box copytopoint to the grid.I find that both @scale and @pscale work,different size of boxes are attached to the point of the grid.But the effect are not totally the same.If I use @ascale,@bscale,@cscale and so on,they all doesn't work.
So what’s the difference between the meaning of @scale and @pscale?

@scale is a vector and @pscale is a float. Using @scale you can specify different scales for different axis, with @pscale all axis sizes remain the same.

Typical practice is to use @pscale for everything!
The only time I have ever had to specify @scale, at the SOP level, was when working with clusters, it is used to set the box size of the cluster.

Try this in your code to observe non-uniform scaling on all axis.

From your answer,it looks like @scale and @pscale have the same meaning,they are both represented the size of the point.The only difference is that one is vector and the other is scalar.Is what I understand above right?

With pscale you set one size. But I think With scale you can set scalex scaley and scalez so you can control those 3 values separately if need be.

you can even use both, they'll be multiplied
You may want to look into docs for more info
https://www.sidefx.com/docs/houdini/copy/instanceattrs.html [www.sidefx.com]