Jim,
You are in the right ballpark with understanding, but not the part that makes it relevant here. The only choices I get for notification are email based and the frequency is instant or digests (which can be daily or weekly). If you choose instant, then you will be notified only once until you visit the forum again.
What makes it relevant here is that I wish there was another option besides email for text messages. I know they are just emails also, but I would not want all site related email to be text messages, just certain things. If I were meeting a few people for lunch and would not be at the computer in the morning, I might set notification to go to text messages. I can't do that now. If someone posted a "see you there" and a few minutes later someone posted to ask if I could bring something, I would not get the second message using the options we have now.