The JOIN command combines similar objects to form a single, unbroken object. You can also use it to create complete circles and ellipses from arcs and elliptical arcs.
In AutoCAD MEP 2023 toolset, JOIN only works on AutoCAD objects, such as lines, polylines, arcs, and splines. However, you can join duct, pipe, cable tray, or conduit segments, as well as plumbing lines, by merging them. You can also connect plumbing lines or schematic lines with the FILLET command.