Then ACF will match the cart attribute named "Purchase Order Reference" with your defined fields and update the custom field's value with whatever value was provided via the order.
Matching a cart attribute with a custom field is done by looking at the "name" provided in your cart attribute fields and comparing it to your custom fields' label. In case you're wondering, this is done without looking at upper-/lowercases, so feel free to mix as you please.
You can also use the field's name as the cart attribute name, if this suits you better, so this approach also links the attribute with the custom field: