When you create a BOM and assign the BOM lines (sub items of BOM) and
once you approve the BOM, it will show you the level zero in the BOM
item and we can view this in item form > reference (tab).
BOM item will show you the LEVEL as ZERO and its BOM lines will show
you level as LEVEL 1. If you have a Sub-BOM items assigned in the BOM
lines, then it will show LEVEL 2 for those Sub-BOM items. like wise it
will fallow the levels.