Xref (external references) user can view another drawing within current drawing which user work in it without actually inserting that other drawing.
How to make external reference file (xref) in the example we main file and other 4 file to be attached as xref do the followings.

1. Open main file  and type in command line ucs and press enter than type w and press enter . 
or  (go to view menu  and select world ucs .)
2. Go to format and click units select your scale and press ok.
3. Go to model space and type in command line xr then attach dwg.(see the video)

Note:- very important. ( the unit scale should be the same in your all xref and main file)

      how to load  XREF files in autocad