This is the "floating selection"(*). When you have a selection and apply a transform to a layer, only the part in the selection is transformed, and left in that floating layer, that you can either anchor (merge back into its source) (Layer ➤ Anchor or Ctrl-H) or create a new layer (Layer ➤ To new layer or Ctrl-Shift-N). You can also apply some tools (move, most notably) to that floating selection before you anchor. Gimp disables the rest of the tools to have you make a decision.
However, this is one case where "Select all" and "Select none" are different. If there is no selection, the transform is applied to the layer directly, there is no floating selection.
(*) Which is admittedly a left-over from a time where Gimp had no layers and from another time where layers where costly.
However, this is one case where "Select all" and "Select none" are different. If there is no selection, the transform is applied to the layer directly, there is no floating selection.
(*) Which is admittedly a left-over from a time where Gimp had no layers and from another time where layers where costly.