In theory the Al Saud family's secretive Allegiance Council, composed ofprinces representing the lines of each of Abdel Aziz's sons, decides on matters of succession.
As king, Salman might choose to nominate Ahmed as crown prince, or Salman might seek to do a deal with the more powerful senior princes represented on the Allegiance Council, in order to ensure their agreement to one of his own sons becoming crown prince.