"The commander is nominated by Crew Planning, when more than one captain forms a part of the crew and takes into account an even spread of commander duties across the fleet" then follows a long paragraph about what happens when a training captain is on the flight but no one is under training or check, basically the trainer has a choice, he can keep command or give it away, Ops to be notified.
(In some airlines, I think I read somewhere Air India was one, could be wrong, it is always the senior captain rostered).