Well, I have my copy of the first "Millionaire" book...and here's the excerpt on questions...
"Questions are grouped in stacks of fifteen questions--the ladder to $1,000,000. The actual stacks to be used in each show are selected daily at random by ABC's Broadcast Standards and Practices representative."
Now, I'm sure that probably has changed, with the advent of Switch the Question. I would think the stacks still exist, and the Switch questions are a special bank themselves.