The following figure shows a drawing containing two folders. Folder1 is nested within Folder 2. The mechanical browser shows two instances of Folder2; Folder 2:1 and Folder 2:2. The instance identifier, the number that is appended to the end of the folder name, uniquely identifies an instance.
Consider Folder 1 that is nested below Folder2:1. This instance is identified as Folder1:1. Notice that Folder 1:1 exists under Folder 2:2 as well. This is because Folder 1 is nested within Folder 2 and whenever an instance of Folder2 is inserted, Folder1 occurs automatically. It can be said that there are two occurrences of the instance Folder1.
Why is it necessary to know the difference between an instance and an occurrence? Because a few operations are occurrence-based, while most operations are instance-based. For example Visibility is occurrence-based. If you right-click a component in the mechanical browser and turn visibility off, only that occurrence is turned off. None of the other occurrences of that instance are affected. If you right-click a component and override its properties (for example, change its color), all occurrences of that instance change color.