How Is The Depth Of Backpacks Measuered?

How Is The Depth Of Backpacks Measuered? Model Height Width Alta 3520′′13′′ Alta 2017.25′′11.25′′ Revel 2519′′12′′ Revel 2017.25′′11.25′′ What does the depth of a bag mean? https://www.youtube.com/watch?v=golXUPhfFSQ What are the dimensions of a backpack? Model Height Width Alta 3520′′13′′ Alta 2017.25′′11.25′′ Revel 2519′′12′′ Revel 2017.25′′11.25′′ How are backpacking backpacks measured? Why bags are measured in