Multi-box packing is a pro feature available on accounts with 4 or more users.
It allows the packer to switch boxes as they scan items so that the contents of each box can be tracked and a packing slip can be generated per box.
Action barcode reference:
BOXNEXT - scan a barcode with this value to move to the next box. If the box has not been created it will be created first, then it will become the active box.
BOXPREV - scanning a barcode with this value will make the previous box number active. ie If you're on box 4, scanning this barcode will move you to box 3.
The action barcodes can be generated by creating a product and assigning the value as the barcode, then printing by clicking the printer icon beside the barcode where it is displayed on the product detail page for the item.