Reading an otherwise excellent UX article today, I got tripped up on this line:
"Remember, mobile users are on the go, they're in transit, and your design should support and respect this."
"Mobile" more accurately refers to the type of device — phone or tablet — not location or ambulatory status. In fact, the mobile user is just as likely to be surfing on a couch as walking down a busy street.
The context that matters, then, is not where the app or website is used, but how the app or website is to be used.
For example, it's safe to assume that a fitness app is more likely to be used on-the-go, whereas an app that streams video is not. Both apps are still defined as mobile because they live on a mobile device, but the use case for each is quite different. Assuming that the video app should be bound by the same constraints as the fitness app is problematic and overly limiting.
There are, of course, apps and websites intended for both on-the-go and sedentary use; these should support the most restrictive context.