What is the basic functionality of these equipment ? What is differnce between PDU & RPP.

The names can be somewhat self describing.

A PDU typically is a large cabinet which can contain a 480 volt to 120/208 volt transformer, along with several branch circuit panelboards, and are used in large computer server room facilities. Some PDU's can be ordered with many available options, such as harmonic filters, voltage and amperage measuring and recording equipment, etc.

A RPP, in the context you have come across, can be a panelboard marketed by the PDU manufacturer as remote branch circuit panel intended to be powered by the PDU.