UOM exists as a column for implementations where an item can have different Primary, Pricing, and Shipping UOMs. The My Lists feature may need a change to write a default of the item's Shipping UOM to the [EC_List_Items] table. This could also support adding different UOMs to list and eventually the cart if this feature is supported on the site (also custom).
The [EC_List_Items] table has a [UOM] column to easily support the addition of UOM as a custom feature to the site. In base, this feature is not set up and so newly created records in the table will have no UOM. The Single List page is set up to not show the UOM column if none of the records in the list have a UOM value.
In the CSV download the UOM column will by default not be set to download (Display Only). If this UOM Custom feature is set up, the UOM Label will need to be set to “Display and Download”.