preamp tube functions of 2 channel dual rectifier

[analog]

I guess only repair guys need to know this but I was wondering what each preamp tube is responsible for in my 2 channel dual rectifier. I know the manual tells you input-5 stages-fx-phase splitter. I was wondering more in depth. Is the 5th stage just for modern? why is the first tube covered and only one half used? and what is the phase splitter? things like that. thanks for any help you can give...