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There are more responsive features that Fusion and many other themes take advantage of. Media queries and other tools for mobile use are becoming more prevalent and important as more and more people use mobile devices for website access.
Fusion allows the use of media queries to help define specific instances where a particular group of theme settings will be used and what some of those might be. In some themes, this can be used to define a stylesheet specific to a particular screen resolution.
Here's an example of a media query used by Fusion. This is for the Tablet: Landscape layout.
only screen and (max-device-width: 1024px) and (orientation:landscape), only screen and (min-width:960px) and (max-width:1199px) and (min-device-width: 1100px)
In a similar vein, here's the Portrait version.
only screen and (max-device-width: 1024px) and (orientation:portrait), only screen and (min-width:768px) and (max-width:959px) and (min-device-width: 1100px)
From this, you can see that these queries are set up for use with particular screen resolutions and widths. There is an excellent post on how media queries work here: Responsive Web Design
Other mobile tools
There are several other tools that can be used for creating device-centric themes. One major player is the Mobile Tools module. This module can help you switch the entire theme to a mobile theme (assuming you can't or don't want to use responsive features). Mobile Tools work with a number of other Drupal modules to integrate the features you need successfully.
Other modules like Panels, and Context can also help you create different layouts based on the needs of the site you're building. Panels actually works well with Mobile Tools, and you can create a specific variant Panel in conjunction with the mobile tools modules. Context also provides for layouts using the Context Layouts.