The row number for each part in the BOM list can be used to access underlying (incorporated) levels if the part contains them. By double-clicking on the part's row number, the operation and material rows for the underlying part will open in the preparation window. At the same time a button called Back to level X appears in the top right corner, which can be used to go back to the previous or higher level. (The X represents the number of the higher level, where 0 is the highest level.)
This is also possible for purchased parts with structures. For structured purchased parts, the part type P appears in italics.