I could not find this information in Unity’s documentation, nor on the whole of the internet. I recall a previous version of Unity having a mouse-hover popup, but this version does not.
On a RectTransform component, the dotted square button is the “Blueprint mode”; it allows you to edit RectTransforms as if they were not rotated and scaled. It enables snapping too.
The R button is the “Raw edit mode”; when enabled, manually editing pivot and anchor values will not counter-adjust the position and size of the rectangle in order to make it stay in place. However, changing the pivot and anchor values by using the “anchor presets” button will counter-adjust as usual.
@Arsd
The “dotted-square button” is “Blueprint mode”.
If you select an element which is rotated, and then try to switch on “Blueprint mode”, you can see the change of the gizmo. You will be able to adjust this element based on its UNROTATED RectTranform. Otherwise, your change will be based on the rotated RectTransform.
The “R button” is “Raw edit mode”.
If you select some element, and then try to change its pivot.
With “Raw edit mode” on, the change will on effect the pivot value,
otherwise, the element will “move” along with the pivot’s change.