This is by way of ANSWERING a question I had about short releiver use:

Q:Can you explain in specific terms how the ROBOT uses the pen? He used one of my short relievers in the 6th inning the other day which was a
surprise to me as I thought the 6th inning would belong to middle relievers.

Answer: Short relievers can be used as early as the 6th inning (even before if bullpen is very delpeted and starter is injured etc.) if the game is on the line and many other factors are in place such as:
Over-all bullpen strength, short/closer strength, next two batters up, base situation, BFA of available relievers, etc.

The closeness of the game and importance of the situation are factors as to whether a pitcher is used as much as the inning.