BoxMargins->{{left, right}, {bottom, top}} specifies the amount of space to leave around the selection. Positive margin specifications increase the spacing around the selection; negative ones decrease it. Horizontal margin specifications are in ems; vertical ones are in x-heights.
The default setting is BoxMargins->{{0, 0}, {0, 0}}.
The placement of a selection can be fine-tuned using an AdjustmentBox.
