How does it work?
Who sits on the board? (three ringers, all branch types represented?)
Do they have set quota?
requirement for branch type?
then eligible officers sorted by score? B+'s, B's, B-'s etc.
I presume you are asking this question now as the Lt - Lt Cdr/Capt RM - Maj RM Boards are due to publish soon. More detail can be found in BR8373 Art 3025 onwards, which is being folded into BR3 (I think) sometime soon.
AD - no need for 1* approval on an A grade, however it is strongly recommended that 3RO advice is sought for an OPG of A to ensure they truly are at that standard*. For a Lt RN the 3RO could be a Capt RN, but it depends on the organisation...
*It's not that the 3RO can only award it, rather a recognition that he/she has a much wider perspective across the entire OJAR 'plot'. ADPRom's annual report (available on the intraweb) has some sharp words about the awarding of 'A' or 'A-', and I know at least 2 sea-going Cdr's who are driving who have never had above a 'B+'!
Officers Promotion Boards starts with a sift. All those in zone (amount time in rank) are written up and OJAR sent to promotions. They sift all those that have NO, NOT YET or Not Eligible (not passed RNFT etc) to a pile that gathers dust. Sorry that is not fair. They go into your file and over time you accrue reports that show a improvement from NO (the first reports) through NOT YET to NOW (with grading). Dependant on a number of factors those that remain are again sifted by a "star" system. Each candidate gets a graded based on the OJARS grades and some other stuff and this is a sort of best on top.
The the board reads all these packs. About 5 years of reports are read through and from this they have a Yes, No and Maybe pile. Then they chat. Hopefully the Yes, No and Maybe piles are similar. Any that don't are discussed.
Lists are drawn and then checked against the number of places (Manpower commitments decided how many Warfare, Pilots, Engineers etc are needed). Slide the top ones over. Pass the list to senior officers to cast an eye. They sign off and a signal goes out.
That is a simplistic version but to answer questions.
It is fair. And no, one bad report (unless a real shocker - say you do something really bad) will not scupper you outright, the board declare interests (the RN is too small so everyone knows everyone but if say you shagged one of the boards wife you would look to having your reports read by someone else).... what else..... no there are no chicken bones involved. And yes there will always be a few on the list that we all say "HTF!!!!!!"