This is the standard UCAS application and is initiated by the applicant. If an applicant applies to two or more courses at one or more UCAS member institutions they pay the full application fee. If they apply for a single course at one institution they pay a reduced application fee. On payment of the balance of the full application fee they can apply to additional courses.
There are two versions of the RPA; one is for home or other EU applicants and one is for non-EU domiciled applicants. RPAs can be used when an institution agrees to accept an applicant who has met the entry requirements for the course. The applicant must understand that they will be made UF at that institution and cannot be considered by any other UCAS institution.
This can only be used when the UCAS institution has submitted an Overseas Partnership Notification Statement (OPNS) to UCAS. It is used when an institution agrees to accept an applicant who has already met the entry requirements and when the applicant understands that they cannot be considered by any other UCAS institution. Further guidance on OPFs is available here.