Additional qualifications and duties for 2/Os include Preliminary Navigating Officer(PNO), Point Defence Officer (PDO) (as mentioned) and Fleet Navigating Officer (FNO). The junior 2/O will normally be PDO, the senior effectively "navigator" regardless of whether his qualification is PNO or FNO.
1st Officers will be titled as combined Navigator/Operations Officer (NavOps), but in reality with a 2/O doing most of the navigation the 1/O concentrates on the Ops role. For career development, 1/Os with a bit of seniority in rank usually train with the RN as either Specialist Navigating Officer (SpecN) or Principal Warfare Officer (PWO) and go on to take a staff/shore/training job using those qualifications. I'm not aware of any in the RFA that have done both, though it's not impossible, especially for any who are ex-RN. It isn't compulsory to complete either of those to reach Chief Officer, but it helps a lot. About 2/3rds or so C/Os are SpecN or PWO.
Chief Officer (Choff/XO) is responsible for cargo operations, heading the deck department, whole ship coordination, and by this time you will have experience across the whole range of activity and specialisations within the department, setting you up for command as a Captain.