This form is accessed from the Block - Copy Layers tool.
Any projects you have open and all their drawing layers will be listed in this form. Highlight the project(s) and drawing layer(s) you want to copy and press OK.
The new drawing layers will be created in your current drawing. (The new drawing layers will have the same names as the originals, so long as the names are not already in use - if they are then the names will be suffixed with _1 etc.)
Copy Drawing Layers From Other Projects