There are times when it is useful to annotate a spot or element in a diagram (especially an analysis diagram like Use Case, Sequence, etc) with a graphical note.
xtUML editor does not currently have a “Note” tool, but the “Activity Partition” tool from the Activity drawer in the Palette can be used to serve the purpose. This tool simply draws a line, which you can add text to.
To make the note stand out, activate the context menu on the newly created element and select Format > Line Color > Red.
The context menu also contains the “Layers” entry. You can add all your diagram notes to a “Notes” layer, then you have the ability to hide or show the Notes layer to show the diagram with the additional annotations or not.