You can move a label for a model element independently of moving the element itself. (I.e. if you move the element, the label keeps the same relative position; but if you move the label, then only the label moves - the element doesn't.)
There is no constraint on where you move the label to, so it is possible for the label to be visually far removed from the element that it is labelling. You can also move the label to be inside the element: e.g. a compartment label inside the compartment, or a submodel label inside the submodel rather than (by default) just outside it. Clicking on either the label or the element will highlight both, which is useful if you reach a situation where it is not obvious which label goes with which element.
To move a label relative to its element, the element itself must not be selected. If the element is selected, then dragging the mouse across the label will select the text of the label, allowing you to edit the label. To move the label instead, click in a blank area of the diagram or choose "Unselect all" from the context menu to ensure that nothing is selected, and then drag the label to its new position.
In: Contents >> Working with model diagrams >> Moving model diagram elements