You've got an existing product that works on a single platform, and you want to allow people to use it on another platform—imagine creating a version of your desktop software for use on a tablet. That’s what Elan Tanzer Lennard writes about in this glamourgeek.com article: “Taking Desktop Software to Mobile Devices."
The key idea behind Ms. Lennard’s article is that you have to make sure that you understand the context of usage of your product. It isn’t sufficient to understand the goals of your users, you have to understand the behaviors and tasks a user is interested in performing on each device: “The first step in determining the functionality to take to a mobile device is understanding the different types of user behaviour for each device.”
This makes a lot of sense . Consider software for making investments: I may want to do extensive research and financial modeling on a computer with a keyboard, I may also want to place trades or review my positions from a mobile device. The reason in this example is immediacy—I may benefit from making a time-sensitive decision when I’m not near a computer, although the improved usability of using a computer is more important when I’m doing more involved work.
Outside-In Software Development, by Carl Kessler and John Sweitzer, is a good book for people who have not yet embraced a customer-centric approach to developing products. Kessler and Sweitzer explore the benefits, rationale, and tradeoffs involved, and they make the case for why this is the better way. Once you've switched to this paradigm, you can extend it: You need to plan, prioritize, and design with an understanding of both your users and the contexts in which they will use your products.
But that only gets you caught up with yesterday. If you want to design products for tomorrow—and you do, because what you design today won't be built before tomorrow—you need to think in terms of multiple contexts of usage.
Christophe Stoll and the precious design consultancy understand that users live in a multi-device world with shifting contexts. From that insight, they developed a collection of patterns of multi-screen behavior that can be used to drive insights about product development across multiple platforms. A user's reality doesn't revolve around your product—your product can become involved in your user’s reality—in more than one context.