It gives you the planets that can load the transport/colony ship, but I have also had confusion during ship loads.  The best I have been able to arrange is that colony ships will be set up to load 3 billion colonists and transports will load 6 billion soldiers.  When it shows the load ship dialogue I can tell whether the default 2.5 value is against a max of 3 or 6.  I have recently done transports with 9 billion soldiers and that works, too.  I have never had a desire to have a bigger colony ship, so that is probably why it works for me.