By default, layers live inside one specific panel. Flip the spanning toggle in the properties pane and the layer moves to a "row layer" array in the editor state — its x-coordinate becomes global across the panels row instead of local to one panel. Each panel renders the slice that falls inside its bounds, so the layer reads as continuous from one panel to the next.
Three common patterns:
Spanning layers can extend beyond a single panel's borders. The selection outline + 8 resize handles + the rotate handle all stay visible and clickable in those overflow areas — they're rendered in an overlay above the panel clipping so they don't get cut off even when the layer extends far to the side.