Rule J2.1(4) requires SI to identify all rounding marks.
'Rounding Mark' is not defined in the RRs.
Where courses are around laid marks, either the diagram or the text description of the course will sufficiently identify rounding marks.
Sometimes where courses are around government buoys or permanently laid marks it may not be obvious.
Do you think that the following Si text is useful?
Marks where the course from the preceding mark to the next mark is not a straight line lying on the required side of the mark are rounding marks.