A butt weld places the two member ends face-to-face in the same plane. For full strength the weld must penetrate the full wall thickness of both members; this is achieved by bevelling the meeting edges (a V- or J-prep) so weld metal can fill from one side, or by welding from both sides for thicker sections.
Butt welds are common when extending stock-length tubes (a 6 m stick of RHS can't make a 7 m gate stile in one piece) or splicing repairs. The butt-weld zone is then ground flush so the join is invisible after powder-coat.
CAD60 drawings call out splice locations as 'BW' on the elevation; the cutting list includes the bevel angle.