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)