Not quite. The A/B switch is not dependent on a matrix. It's dependent on a loaded patch. By loaded, that means a patch resident in a cell (the little squares with the cryptic letters).
And that A/B Switch is useless unless an A/B patch is "live" (highlighted, to be performed).
A brief tutorial: using a Vienna instruments patch that is under the DYNAMICS heading (in the Patch list), find an item for something that says "dyn-me" or "dyn-str." Those stand for dynamic medium and dynamic strong -- VI's abbreviation for a patch that has both the sound of an instrument getting louder and softer.
It can't play both at the same time. So they are A - B'ed using the chosen data (usually a CC or a note trigger) as selected in Basic View.
Go into Advanced View (click on the "Vienna Instruments Pro" heading to alternate between Basic and Advanced View).
In Advanced View, select the patch slot with the loaded dyn patch.
Select the "Advanced" tab (fifth tab on the right).
There you'll see a heading called "AB Mode" with three options.
"Global" lets you alternate between A and B as triggered by the setting in Basic View.
"Force A" means no matter what you set or trigger, you'll just hear A (just crescendo in this case).
"Force B" means no matter what you set or trigger, you'll just hear B (just diminuendo in this case).
Okay, now load something like a long patch, or a short -- pretty much anything that's not a dyn patch.
Go to the same tab "Advanced." Next to AB mode, now you've got "N/A."
And that brings us back to your question: anything with "N/A" is indeed "useless."
Understandably, you may be confusing this with choosing two different sounds on the X-Y axis. But as you can see, an AB patch is more granular. It only works on the patch/cell level.
It can be further confusing because that AB-Switch setting under the Y-axis is there all the time, even though it only works on specific patches (and you may not have any of those in a given matrix).