You are only allowed a maximum of 3 backpacks , they act as extra inventory slots , when you first buy a backpack , it gives you an extra 500 slots , so a few stacks extra . You can spend tokens on each backpack to make the backpack "bigger" so you ideally have a bigger inventory . When you are mining , the blocks are automatically put into the backpacks , until they are all filled and then the blocks would go to your inventory . When you sell at the golem , all the blocks including in the backpack are sold , if you have the auto sell enchant on your pick , you can automatically sell what's inside your backpacks depending on the level (pretty sure of this)
btw if you have more than 3 then the forth or higher will not work ,
that...