Levels of Subpictures

Level 1 is the top-level subpicture, which is a combination of level 2 subpictures. The level 1 subpicture is the owner of the level 2 subpictures below. Equally, the level 2 subpicture owns a number of level 3 subpictures. It is an important fact, that all information added to a drawing will end-up in a level 3 subpicture (which is a part of a level 2 subpicture, which is a part of a level 1 subpicture, which is a part of the drawing).