I looked up the local var, PT2,NPT2 means append 2 for the second source.
But I still can't understand what does it mean, and how to use it, anyone can give me a example?

It's used to mean the corresponding thing except in the second input. Say you have two SOPs which have the same point count but different point positions. Then to copy the point positions from one SOP to the other, you can wire them into the Point SOP and then change $TX,$TY,$TZ to $TX2,$TY2,$TZ2. There are various other uses for this since those local variables from the second input can be used to compute other attributes (say normals, etc) on the first input.

Sometimes, I use it to visualize point position differences to see if they're the same by putting in $TX2-$TX, $TY2-$TY, $TZ2-$TZ in the Colour parameter.

thanks edward