[Graham]

How i see it is:
1 event - Rally
Instead of classes A,B,C,D there are now 4 eras of rally.
Inside each era is now 3 classes (A,B,C)
So you pick two eras of rally to enter in, within that, the car you enter can be either class A,B or C
You race as an era but you can be the only Class A car entered in that era and then you win that class.
For example:
1car entered for modern rally, then that car can either be: standard rtr scalextric, Ninco etc (Class A)
Modified class A or scratch built (Class B)
Or a ‘Pro Built’ car NSR, Slot-it etc (Class C)
The whole era runs together and my class A car might come last in the era, but if it’s the only class A car entered in that specific era, I win ‘Modern Rally Class A’


That’s how I see it anyway

Although the terminology has changed from era's called Classes to being called Events (which is probably more correct anyway)
For each Event you have the 3 types of slot car construction (defined by Class type) rather like real sports car racing.

This saves me having to specify different rules for cars in different Era's (Events) in order to accommodate the diverse range of Entrants we get. Often Entrants would prefer to enter a particular "Event" but don't have the right type of slot car. And it is a lottery for me to decide which type of slot car should be associated with which Event. 
There may well be Events with only one or two Car Classes represented, I have no way of judging this as yet, so it will be interesting to see how this works out.

And as Mike imply's it does offer the opportunity for those perhaps without eg. "Pro cars" to compete on a more level playing field than before. But of course you can only win or loose now. No medals or prizes for a near miss.