In RIMMF, a “sub-element” is an element that has another non-root element as its immediate parent.
In the following snap
The indentations used in the RIMMF interface supply a visual indicator of whether something is a sub-element or not.
The collapse/expand indicator (to the left of Title and Publication statement) indicates whether an element has sub-elements. (If there are no sub-elements, then there is nothing to collapse or expand, and that indicator will not appear).
By default, RIMMF places a constraint on sub-elements. This means, we can't grab the Publisher's name, for example, and move it into the Title group.
But sometimes we need to be free of this constraint. To do this, go to the Options menu and select Remove element constraints
Once that's been done, we are free to move sub-elements as needed.