You'll find the list of action barcodes in Settings > System > Scan and Pack near the bottom of the page.

When the switch for an action is enabled and the barcode value specified in the field is entered during scanning, the action will be triggered.

The action can be anything from opening a window to add a note to the order, to removing the currently suggested item from the order.

Each possible action is described in the tooltip beside the printer icon.

 The printer icon unsurprisingly can be used to print the barcode that when scanned will trigger the action.

It's also possible to use macros or devices like the Stream Deck to trigger these actions.