An unlimited data plan seems like a perfect match for the iPhone’s Personal Hotspot feature. The million-dollar question is, does unlimited data include hotspot? The answer is that it often does — but with some very significant limitations.

Tethering is the general name for a feature that Apple calls Personal Hotspot. It lets you share your iPhone’s cellular data connection with other devices to get them online. Most major carriers include tethering in their monthly plans at no extra charge. Better yet, most carriers offer at least some plans that include unlimited monthly data.

Yes, unlimited data plans usually include a personal hotspot.

Sounds too good to be true, right? As if there had to be a catch? Well, there is.