Using normal RKCB where the intended trump suit is one of the minors can be difficult, in that an unfortunate response may take the partnership beyond a 5-level contract in the suit in question. So a bid of 4♣ or 4♦ is usually the RKCB asking bid in that suit. About the only time when this isn’t the case is where the bid is made as part of a competitive auction.
These are exactly as per normal RKCB, except they are made at a lower level. So, if the asking bid is 4♣, the responses are:
The RKCB enquirer can still go on to make further enquiries, e.g. to ascertain whether responder has the lower holding [one or zero] or the higher holding [four or three] following either of the ambiguous responses. There is a difference here from standard RKCB, especially playing pairs rather than teams, and that is that the RKCB bidder enquires with 4NT rather than five of the minor. This allows the partnership to play in a no-trump contract, albeit at the four-level, if they abandon their slam quest, rather than losing out to the 3NT bidders.
Otherwise, subsequent enquiries have the same meaning as in standard RKCB, but again at a lower level.
One consequence of using minor-suit RKCB is that Gerber is consigned to the foul-smelling pit in which it belongs. A direct bid of 4♣/♦ over an opening 1NT/2NT becomes RKCB in the suit bid