Now X has several new aspects designed to allow you to easily configure the product. Some of these ideas were present in Now Up-to-Date and Contact, but in Now X we've taken these concepts a little further. So, here is an explanation of the hierarchy and relationship between Tabs, Sets, and Styles.
A Window contains one or more Tabs.
A Tab controls what panels are displayed: whether the category, tag, activity, mini-month, task list, or inline detail view panels are displayed; which panel is the main panel in the middle of the window; what the size of the window and the various panels within it are, and what the current search text is for filtering.
A Tab also has one Set associated with it. The Set controls which categories and tags are turned on or off, which controls what data is displayed.
A Tab also has one Style associated with it. The Style controls how data is displayed: what font typeface and size are used; whether day names are shown; what columns are displayed in lists and what the sort order is; background colors for various types of day cells; what event types are displayed; whether word wrapping is turned on or off, etc. Basically, anything set in the Properties panels are stored in the Style.
In previous versions of Now X, all this data was stored in a "Tab", which we found was not flexible enough to accommodate having multiple workspace windows open. If everything was stored in a Tab, you'd have to recreate the list of categories you want to see data for, plus all your panel properties every time you opened a new window. But, with separate Sets and Styles, you can reuse this information in multiple workspace windows much more conveniently. The only structure that does not cross window boundaries is the Tab itself, which belongs to only one Window at a time.