An Extend Annotation indicates that this method/tcm extends a previous layer. A previous layer is in the same struct/unit, in the load order, without considering its kind (is only / is also ...) and without considering when sub-types.

Extend annotations are rectangles on the vertical ruler to the left of the editor, next to the method layer:

A green rectangle indicates an extended method

Hover an extend annotation to see the previous layer.

Click on an extend annotation to jump to the previous layer.