Normally, all Max for Live devices within a session reside in a shared Max universe.
As such, any objects that reference other objects by name such as buffer~, groove~, cycle~, lookup~, send, receive etc will share that name space with other Max for Live devices within a session.
By adding three dashes (i.e. “---“) directly before the name of a buffer~ etc, that name is then only used within the scope of that particular instance of a Max for Live device. This is because the three dashes are replaced with a three digit number unique to that particular instance. All occurrences of three dashes within a given Max for Live device instance will be replaced in the same way.