What is a backpack in the military?

Excuse me, could you please clarify what you mean by "a backpack in the military"? Are you referring to the standard issue rucksack or backpack that soldiers carry during field operations, which is designed to hold essential gear and supplies? Or perhaps you're inquiring about a specific type of backpack used by military personnel for a particular purpose, such as a tactical backpack or a survival backpack? I'd be happy to provide more information if you could elaborate on your question.