Drag and Drop
Drag and drop moves an element that already exists. It is TYPO3's own Page Module mechanism: grab a content element by its header and drop it where it should go. Grid Elements extends this mechanism so it also works across a grid's own cells and into and out of nested structures, honoring the same restrictions that govern any other way of placing content there.
This is a different action from the Drag-In Wizard. Drag and drop always moves or copies a content element that is already on the page. Creating a brand new element by dragging its type directly into a structural area is the Drag-In Wizard's job, not ordinary drag and drop, even though both interactions look similar at a glance.
Moving within a Grid Element
Dragging a child from one cell of a Grid Element to another cell of the same Grid Element relocates it there. Only cells that would actually accept the dragged element highlight as valid targets while dragging; a cell whose restrictions rule out that element's type simply never lights up, so an invalid drop is not offered in the first place rather than rejected afterwards.
Moving between structural areas
The same drag works between a page column and a Grid Element's cell in either direction, and between the cells of two different Grid Elements on the same page, including cells of a Grid Element nested inside another one. Dropping an element into a plain page column detaches it from any Grid Element it used to belong to; it becomes an ordinary page-level element again, placed in that column like any other. See Data Model if the underlying field changes that make this possible are of interest.
Copy instead of move
Holding down Ctrl while dropping (Option on macOS) copies the element to the target cell instead of moving it, leaving the original in place. This is the same modifier TYPO3 uses for drag and drop elsewhere in the Page Module.
Restrictions decide what is offered
A cell's allowed, disallowed and maxitems configuration is
checked while dragging, not only after dropping. Cells that would
reject the dragged element's type, or that have already reached their
maximum number of elements, do not become active drop targets at all.
See Restrictions, Validation & Permissions for how these rules are configured and how
they are enforced again, independently, once a drop is submitted.