With Show as per Condition FrameMaker will show all text which has A applied, no matter, if other conditions are applied as well. This is the OR logic.
If A should be shown and AB should be hidden, you must use this expression:
A will be shown.
Then the expression checks for B. and hides all text with B.
I admit I experiment, if I want to achieve a specific result, specifically when there are more conditions involved.
Probably this is easier in FrameMaker 12 as it allows to use brackets.
If by "the original FM condition tag approach", you mean the Show as Per Condition Show/Hide shuttle boxes, then I do not believe there is a way to specify "OR logic" based on the hidden tags. Before the introduction of conditional expressions, FM always showed conditional text if any of the applied tags were shown, as Show as per Condition still does. I agree with Winfried on this one.
Thank you all very much for your replies. Winfried's approach worked. Sorry, Arnis, the others seem to be right about how the basic logic works. Simply telling FM to show A and hide B does not hide text that has both conditions applied. The ANDNOT expression Winfried provided does work, however. Thanks again!