Glossary » working set
a small number of resources that a user switches between frequently, such as files users frequently edit or drawing tools they rapidly alternate between. The working set is identified so that the interface can support moving quickly between items in the current set while more slowly accessing less common items.
A bookmark list is an example of supporting frequency, but a working set is meant to emphasize optimizing your current task with only a small set of items, say, 2 to 6. The Windows taskbar is an example of giving the user easy access to a small number of current tasks, but doesn’t continue to support it well when your number of open files is large.