When Apple announced the new iPhone XS, XS Max, and XR on Sept. 12, one particular feature captured the imagination of international travelers and multiple phone-owners alike: the dual SIM.  

If the processor is your phone's brain, the SIM, or subscriber identity module, is its soul -- a tiny, little chip that stores your phone number and connects you to a cellular network. Essentially, it's what makes your phone your phone.  

If you've ever wanted two phone lines on one device, Apple's new Dual SIM Dual Standby technology could be a life-changer. It will make it much more practical to have two phone numbers on one phone -- one for business, another for personal use. And it will make it considerably easier and less expensive to use your own phone when traveling abroad.  

Though the beta release of iOS 12.1 includes a crucial component of the technology -- eSIM remote provisioning -- Apple won't deliver it to the general population until later this fall. Of course, it won't be the primary factor for the vast majority of people shelling out for a new iPhone ($999 at Sprint) in 2018, but the debut of the iPhone XS, iPhone XS Max and iPhone XR marks a new era of dual-SIM phones going mainstream.  

We don't yet know all of the details about how, when and where Apple's dual-SIM feature will work. But based on our current understanding of how the technology will be implemented, there are some limitations. For now, here's what we know about Apple's dual-SIM -- and what it means for the three new iPhones.