Group changes how you select objects
An uploaded layered SVG appears grouped in Design Space. Ungroup it to move or resize its layers separately. Grouping is not the same as welding contours.
Attach keeps placement
Attach holds objects in their Canvas arrangement for the mat. It can also associate a draw or score layer with a cut layer.
Flatten is for a printable layer
For a multi-layer Print Then Cut design, use Flatten to make a single printable layer with an outer cut shape.