A mitre joint is the standard corner detail in gate frames where both members are the same RHS or SHS profile. Both members get cut at 45 degrees on a cold saw or band-saw mitre bench, butted together, and welded full-perimeter to form a square corner with no end-grain visible.
Mitre joints are stronger and cleaner than butt joints (where one member runs through and the other lands on its face) because the weld runs around all four sides of the tube section rather than just three. They also look cleaner in finished, painted gates. No visible weld sits on a hard edge.
CAD60 cuts the corners of a rectangular gate, fence panel or screen frame as true 45 degree mitres by default: the 3D view and the STEP model show the angled ends, a thin line marks each mitred corner on the front elevation, and the cutting list gives every frame member at its long-point length with the 45° cut, ready to feed straight into the workshop saw. Choose Butt under Fine adjustments if you build the frame with square-cut rails and stiles instead. Where a mitre is not possible (a different tube for the bottom rail, round or angle section, a raked or slanted top) that corner stays a butt joint.