Action code 135 can be used to split up fleets by build code. The action is a move and do meaning the fleet will move to location #1 before splitting off the ships into an existing fleet or a new fleet. If it is an existing fleet then the two fleets much end up in the same location after the fleet moves.
However this action also works without a move attached. If you don't include a location then the fleet stays put and simply transfers the ships. This is useful if you find yourself with more than 50 ships in a fleet. Action will still work because the fleet doesn't actually move so the move restriction is never checked.