‘As these examples illustrate, tab-bar navigation is ideal for letting the user choose among modes of operation tailored to specific features, information, or even states of mind (“I can’t think of a restaurant, pick one for me”). A tab bar summarizes what your app does; it’s a series of miniature advertisements for how the app can help. Tab bars make for efficient navigation, since they take people directly to specfic screens to perform specific actions, but they require careful planning by the designer. The tab bar should offer a set of options that match up neatly to specific and common missions your audience has in mind when they launch your app.’
Clark 2010, p.108
Slight confusion of tab bar and toolbar!