The Union button unites coaxial entities.
Quick Access Panel: 
Ribbon: Editor -> Instruments -> Union
Menu: Tools -> Modify -> Union
Select coaxial entities holding the Shift button and press the Union button.
Example: coaxial entities
before: 
after: 
Go to ABViewer
URL of this topic:
https://cadsofttools.de/help/en/index.html?tools_menu_union.htm