Sub number is used for Kits or Configurable items. A kit is a collection of items which can be sold for a different price than the sum of the individual items. So a Pack of 3 thingys might be sold for $12 whereas 3 x thingys might be sold for $5 each. You cannot use that field for the purpose you desire.
Your use-case is precisely what Sales Item Characteristics are intended for. Set up any number of custom Characteristics (and you can name them Shift1, Shift 2 etc. and use those characteristics to influence the Packing List Batch to separate out the order deliveries.