In Superchess, while promoting a pawn, it is only allowed to promote to an available piece. That means either a piece that has been captured earlier in the game, or a FIDE piece that you didn't use because it was replaced by a variant piece.
Per game or variant this should be stated explicitly, to what extent you are allowed to promote to any piece you like.
1. You may consult chess books, periodicals, and written online resources.
2. You may not use computer programs or endgame tablebases unless you and your opponent have agreed to prior to the game.
3. You may not consult other players for advice unless you and your opponent have agreed to prior to the game.
4. Announce moves using algebraic notation unless you and your opponent agree to another system. Ensure there is a non-ambiguous designation for each piece (i.e "Ar = Archbishop, Ad = Advisor").
5. Some variants may have other rules which require supplemental information. In these cases include information to ensure the declared move is clear and non-ambiguous. Example: A variant may have the king and queen starting in reversed positions compared to FIDE chess, so when castling it may be necessary to add information such as "28.0-0-0 (long side castle)", to ensure it is not taken as a queen-side castle.
30.Rd8
conditional:
If...RxR then 31.QxR
As with any other move, conditional moves are binding and cannot be changed. The opponent may reply to the entire sequence, or he may play each move at normal time control.
9. Missing or mistaken announcement of check, capture, or "e.p." does not invalidate a move.
10. A declared move may be accepted as valid if it has capitization errors but is otherwise non-ambiguous. (example: 12.B3 may be interpreted as 12.b3, a pawn move).
11. A move announcement has priority over any commentary submitted with a move.
14. If your opponent submits a diagram along with the move notation, check the diagram for correctness before proceeding. If the diagram has an error, notify your opponent. A valid move notation has priority over a diagram. An error in a diagram is not by itself a reason to disqualify a game if all posted moves are legal. Play continues even if a diagram has an error for any number of moves. (Therefore if you are using an opponent's posted diagram it is important to check it for correctness before using it).
15. Time control: Three days maximum per move, unless agreed otherwise. Once notified of an illegal or ambiguous move, the time spent correcting or clarifying the move must be within the three day day limit. If your opponent has not announced his move within the three day limit, request a reply. If a legal move is not announced within one extra day (24 hours), your opponent is a "no show" and he/she has forfeited the game. A maximum of three time-warnings are allowed per player every 40 moves. The fourth time-warning is a loss. Every 40 moves the limit is refreshed.
18. Board updates: Unless otherwise agreed, one player in each game should post a board update, or a board should be provided by someone who has agreed to moderate the game. Either player has a right to request a board once per move (2 plies). If a board has not been posted which is at most 1-ply from the current position, and a player requests to see a board, time waiting for the board does not count as playing time.
Section-3 (Non-players of a game)
19. Do not post comments about an active game on any website or public forum.
Note: some discussion related to revisions of these rules were agreed in the bulldog club (here).