Blocks are organized on the Canvas in a hierarchy. A list view of this hierarchy is visible by selecting the Page Layers tab in the toolbox. This can make selecting Blocks simpler and easier as some Applications may have many nested layers and it can be difficult to select them on the Canvas itself. Each Block that is selected also displays a list of any children Blocks if applicable. This allows you to find the exact Block you need to manipulate.
It is recommended that you read the article listed below to improve your understanding of Page Layers.