Any shooting position is allowed, but some lanes may be designated as standing or kneeling. in the event of a competitor not being able to conform to the above disciplines the following shall apply
taken from bfta shoot rules 10 i
this is taken from the top of the photos showing the adapted positions "it is neither a new rule nor an amendment to the exiting rule."
if you take both together it seems to me that the first rules makes it quiet clear that if you cant conform to both disciplines that is when the adapted positions come into play as would the clarification that the adapted positions are neither a new rule nor an amendment. the first quote states disciplines i:e plural that to me means that you must be unable to do both of the positional s before being allowed to take the adapted positions up