Rp
This element is normally used with ruby and rt in order to show the oriental pronunciation. More specifically, <rp> tag contains what the browser will show in case that it does not work correctly with ruby tag.
r
Rt
This tag has been designed to be used along with ruby tag, as a descriptive complement of the oriental pronunciation.
Ruby
It is normally used with rt and rp tags, to describe oriental typo. For instance, it is very useful to define an asiatic typo within a dictionary with its pronunciation.