External Reference Xref

External Reference is a link of another drawing and be able to see all the content in the current drawing

View tab > Palettes panel > External Reference Palettes

Attach DWG

Navigate to the drawing location

Reference Type: Attachment

Path Type: 
Full: Drive\Folder\DWG
Reletive: Folder\DWG
No:  DWG

Layer:
A "|" symbol indicate that this layer from which external drawing

External References palette - Right-click the Xref - Bind

Bind Type: Bind

After bond
A "$0$" symbol indicate that this layer previously from which drawing