The last code upgrade requires 1/0 SER cable for a 100 amp sub panel. I vaguely remember it being mentioned in code update class, but I can't seem to pinpoint the specific code area where this changed. Okay I forgot! A #1 used to be okay. I know it was changed, but I would like to see where or why it was changed.

Please try to be specific with this question...
What year code? 2011? or 2014?
Is this residential? Commercial?
What is the system voltage? 208 volts or 240 volts? 3-phase or 1-phase?

All of the above may make a difference in the minimum requirements for
cable or conductor ampacity.

I believe you are asking about 338.10(A)(4).

This applies to the use of type SE cables for interior wiring. But it was not rewritten in NEC 2014.

Actually Don, this may be about 310.15(B)(7), which has been re-structured. After you look at it and do the math it really doesn't change much, although you do have have to verify case by case.

Agreed, 310.15(B)(7) caught my eye when the pre-release was sent to me.

Once you do the math and look at the previous you see you end up at the same conductor size.

I guess they just wanted to exercise the, "little gray cells", to quote Agatha Christie's Hercule Poirot

yeah, we call it the 83% solution...