When elements overlap, the one drawn last is in front. The draw order decides which that is - and, inside a single element, which of its contents is in front of the others.

Changing the order of elements
Arrange	On the panel's strip: Bring all selected elements to front or Send all selected elements to back. The selected elements keep their order among themselves.
Right-click an element	Bring to front or Send to back, for that element only.

Elements are ordered within their own label set: an element of one label set is never in front of or behind an element of another.

Page elements are drawn over the labels or behind them, as their Position says, in the Page element panel. Bring to front puts a page element on top of the labels, and Send to back behind them.

Inside an element. An element's contents are drawn one over the other. Normally the barcode is in front, then the text, the line, the grid, the image, the pattern and the border behind them. To change that, open Advanced in the element's Properties and drag the contents up or down in Components order: the top of the list is in front. The background is always at the very back.