Question re: MLB umpiring

[gibson]

Need help answering a question, please.

How are stations determined for a series of games in MLB? How do they determine who the plate umpire is and who the field umpires are? How are these umpires rotated? Who makes those decisions?

Have looked in the MLB Rulesbook, but I couldn't find anything about umpire stations being rotated. I told a friend that they do because I can point to any number of games I've been to where this will happen from one game to the next, but I have no idea what rhyme or reason is behind this.

Please help.

Thanks very much.