So, I don't use Target, but I do support A/B testing (we use a different tool).
Technically a conversion is just a special metric (in the context of your test).
For instance... let's say you are doing a test on a click on a container (maybe this container just expands and collapses):
In this case, you would want to track the click as a conversion (maybe specifically expand actions)
Or maybe you are doing a test that takes users through a purchase flow.... you may want your conversion on the click of "make payment" (s.tl / action call) or maybe you want the conversion to track on the "thank you page" (s.t / page view call) after a purchase is completed.
These are decisions that you will have to determine based on how your site works and what type of test you are running.
I would assume (possibly incorrectly) that when you choose a Page View as a conversion, you can target specific page views (i.e. thank you page PVs) rather than just any page view...
Hopefully someone else here can support the Target specific questions...
But I suspect if your mbox clicks don't resolve to a page view, then your conversions wouldn't increment (something like the expand and collapse example above, the user stays on the same page).