It is divided in two tabs:
The CSS tab, where there is specific CSS stuff, divided in two parts:
The selector scanner that helps in validating syntax in a single selector, and lets you edit values from the list either manually or by double-clicking on the arrows.
The CSS tree view where all the existing CSS2 properties and values are shown and can be selected for insertion in the document by double-clicking on a value.
The Digest tab, which allows you to parse the current document by selectors.
It offers a convenient way to access quickly a given selector in a big document.