Please Explain the difference of Xref (External Reference) command in AutoCAD and 3D Studio MAX.

Xref is a simple drawing inserted in current drawing with XREF command. Any dwg can be a xref if you use XREF for insert in current drawing, or Block if you use INSERT command: main difference is memory, with block yout current drawing increase memory space because contain drawing while with xref you have small memory because drawing is external and you use only xref name, insertion point, scale factor and rotation angle ie. (<100 KB) for inser drawing. 

3d studio max was the primary software I used. The program can be used for so many things from character modeling and animation to particle systems like renders of fire, smoke, and water. One of my favorite uses of the program is the reactor system which lets you set physical constraints on objects and the system will figure out how they interact with each other. The easiest example of using reactor is creating a brick wall. You place every brick and then crash something into it and the program calculates how each brick would react. This would take absolutely forever to figure out on your own.