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You're on the right track. Resizable is the only interaction currently that has an individual css file. The others don't need them because they don't have default styling, as they're interactions, more like behaviors, that can be applied to most any element, in contrast to widgets.

To answer the rest of your question, there are two main groups of classes in the jQuery UI CSS Framework:

1. plugin-specific classes (begin with .ui-pluginname- prefix)
These classes allow you to target specific parts of specific widgets.

2. generic classes (begin with .ui- prefix)
These classes allow you to target generic parts or even states of all widgets.

The plugin-specific classes are what you'll find in individual plugin css files and deal mostly with functional or layout css.

The generic classes are further split between jquery.ui.core.css and jquery.ui.theme.css.
* The core css contains only functional and layout css: no fonts, no colors, no borders, no backgrounds, no images.
* The theme css contains all the look-and-feel css: fonts, color, borders, backgrounds, and images.

This allows someone that wants to build on the jQuery UI CSS Framework by creating their own custom theme css to completely change the look and feel without throwing away some of the utility css in the core file.