From memory, I think that Dog Cow is mostly right. However there were some non-ROM differences between the 00 and 01 boards -- ADB and timing corrections? Most of the time, applications will not notice the difference.
My Woz IIgs came from an enthusiast who ran the latest and greatest stuff on it. It boots up to declare itself as a ROM 01, but like most like it, it's an original board with ROM swap.