Comment by wat10000

In theory: if you’re clicking on a UI element that has some notion of being selected, then a single-click selects it, and you need a double-click to take an action on it. If there’s no notion of selection, then a single click takes an action.

In practice: adherence to this ranges from perfect to abysmal. And users who don’t understand the computer well may not know how to think about whether a given UI element is selectable or not.