The Select All command, as its name implies, selects all entities and is subject to the current selection mask and the show/hide status of the entities. For example, if the selection mask is set to only allow selection of domains, Select All will pick all domains that are currently not hidden.
Tip: The Select All command can be accessed in multiple ways. You can use the context menus, the Select, Select All menu command, or the Ctrl+A shortcut.