Unfortunately, it's seldom made very clear, and when you have the meeting with the umps before the game, you often get a different answer to the question every game. Somehow over the years, the reasons for courtesy runners seems to have changed from a rule to speed up the game allowing the pitcher and/or catcher to get ready for the next inning, to more of a strategic move to put someone faster on base and/or to lessen the chance of injury to a pitcher or catcher, either of which I am OK with but I just hate all of the confusion. I bet we had 10 games this year where we actually had to stop the game for one team or the other to get a clarification as to who they were allowed to bring in, which totally defeats the original purpose. If the purpose is to speed up the game, then why not just allow it to be the last out?