An order of battle is a list of the various elements of a military formation organised within a hierarchical command structure. It can also provide information on the strength of that formation and the equipment used. An order of battle is not necessarily a set structure, and it can change depending on tactical or strategic developments, or the evolution of military doctrine. For example, a division could be altered radically from one campaign to another through the adding or removing of subunits, but retain its identity and prior history. The size of a division can vary dramatically as a result of what forces are assigned and the doctrine employed at that time.