AT&T has a new optional feature for some of its plans. It’s called Turbo, and for $7 per month, it provides “better speed and stability” for a line of service by upgrading your data plan to “performance data.” AT&T pitches it as an add-on to help with demanding applications, like gaming. Okay, but what exactly is “performance data?” It’s kind of unclear. But we can sort of piece it together based on what it isn’t.

If you were hoping Turbo could help boost service on a prepaid or entry-level postpaid plan, I have bad news. It’s only available on a handful of AT&T’s postpaid plans: Unlimited Premium PL, Unlimited Extra EL, and Unlimited Elite. They’re already some of AT&T’s fanciest and priciest plans, with lots of hotspot data and “premium data” that isn’t subject to slowdowns when the network is busy. Additionally, Turbo only applies to your plan’s included data allotment — if you run out of hotspot or premium data, you’re still subject to slower speeds.

Turbo isn’t making use of next-gen technology like L4S to make connections more stable, either. According to Erin Scarborough, senior vice president of consumer product for AT&T, “L4S technology is not used today but we do plan to continue to advance and evolve AT&T Turbo in the future.”

The Mobile Report suggests that Turbo just bumps customers up to a higher tier of service, or what’s known as Quality of Service Class Indicator (QCI). Under the hood, wireless networks assign these priority tiers to different kinds of network traffic. Some AT&T customers worked out that their wireless data QCI levels were recently bumped down a tier. So is AT&T just selling people back speed that used to be included in their plans?

AT&T, unsurprisingly, claims that’s not the case. Scarborough explains: “Although AT&T Turbo currently is assigned to a QCI to which some of our consumer traffic previously was assigned, we’ve materially modified it and increased network resources and relative weighting for AT&T Turbo traffic, thereby creating a higher level of performance than we’ve ever before offered to consumers.” Fine.

That still doesn’t completely explain what performance data is, but if nothing else, the whole thing is a reminder that there’s always more to your wireless carrier’s terms than meets the eye. Don’t even get me started on “unlimited.”