A unit of measure can be defined for an item and used to filter and sort the item list and reports. For example, a sweet store can define units of measure in "ounces" OR "lbs" and then assign one of those units to each of the sweet.
Under company preference on Inventory tab, the Unit of Measure can be defined.

Multiple Units of Measure (In Pro / Multi-Store Edition of Point of Sale – POS):
With the use of units of measure (multiple), one can buy and sell items by different units. For example, if an item is bought by the case / pack / box but can be sold by the bottle, or bought in the bulk but sold by the pound.
Under company preference on Inventory tab, choose I Want to set up multiple units of measure. One can buy and sell an item in up to four different units, using Unit of Measure (Multiple).
A separate UPC (unique product code), ALU (alternate lookup), sales price, and default purchase by and sell by units in inventory can be defined for items in Point of Sale – POS and the correct unit of the item is suggested on the documents.
When the feature of multiple unit of measure is turned off, the single unit of measure capability is still retained.
For an Item, the Alternate Lookup (ALU), Item Number and Universal/Unique Product Code (UPC) are referred to as Item Identifier in Point of Sale – POS.
Alternate Lookup (ALU): It is an optional field for an item that can be used to search, list, and print items on documents. It is more suitable for retailers who don’t tag items or those who want to print documents, such as purchase orders, with alphanumeric product identifiers (such as a vendor’s part number).This field can have text descriptions, catalog/part numbers, alphanumeric entries, abbreviated names etc. that allows to quickly locate an item. It should be unique for each item for which they are defined.
Item Number: Except Special Order Items, every Item has an Item Number in the list of items. Item Number can be edited but not deleted. Character Length of Item Number is from 1 through 999,999, 999. Item numbers above 999,999 are not advised or suggested as the bar code would become too large to fit on the default tag template. Item numbers are used to print bar codes on tags in Point of Sale – POS.
Universal/Unique Product Code (UPC): There are pre printed tags from the manufacturer of the Item where UPC is already recorded in the UPC field of the Item. UPCs should be unique and contain numbers only. The UPC is normally recorded in the UPC field as 6 digits UPC-E / 12 digits UPC-A / 13 digits EAN or likewise identifiers up to 18 digits. Point of Sale - POS uses a standard conversion algorithm to convert an UPC format less than 13 digits to a 13-digit form.UPC numbers can be manually recorded or the preprinted tags from vendors can be scanned into the UPC field.
Important: Bar codes created in Point of Sale - POS from the Item Number should not be scanned into the UPC field.