A Bag Inventory is a listing of the part contents of a set. It breaks the parts down by what bag they come in. Bag Inventories were important for resolving the bagging used in LDD 1.4 LEGO Factory palettes.
In general, a bag inventory may not be definitive. For example set 5542 is known to come in at least three different bag breakdowns, speculated to be due to having been produced in differing production runs, with the parts baggers set up differently.