Impress and Draw are the same application what a slightly different UI. In both, you play with drawing objects, i.e. a shape with a label. Most of them are rectangles, e.g. slide title and slide main text.
The label in these objects is managed “locally”, inside the shape, without interaction possibility with other shapes. The shapes are z-ordered, each one in its own layer (yo have as many layers as shapes/objects). Since the layers are “infinite” and populated with a single object, this object is always drawn as if alone. z-ordering just tells in which order to paint the shapes. Objects deeper are hidden by shallower ones.