I've had several B2B test message codes on my account over the years, it does NOT mean it only counts for messages send to other business accounts, it's for ANY text message you send/receive. How would they know? I get text messages from my servers at work and technically those are "Business", but the source isn't from a "Business" account.
The way it was explained to me was that the "Business" texts are considered AT&T to AT&T texts. They can easily track which texts are in network and which are out. I'm not defending this idea, I certainly hope it is wrong.