It the hypothetical example described it could be anything -- in fact it could be exactly like the "borders of Canada", some stretch of land with guards and fences. Or it could be some "portal", a mirror you can go through, a black hole, whatever.
That place would just have to comply with a different set of rules compared to anywhere else to be justified to call it an "alternate reality" (e.g. no gravity, fire is cold, etc).
Heck, we even call experiences like living with the Amish an "alternative reality", and those are on our very same universe/world, and with all the known laws of physics and basics intact.
That place would just have to comply with a different set of rules compared to anywhere else to be justified to call it an "alternate reality" (e.g. no gravity, fire is cold, etc).
Heck, we even call experiences like living with the Amish an "alternative reality", and those are on our very same universe/world, and with all the known laws of physics and basics intact.