Product Quantity Settings
These settings allow us to add custom quantity label to places wherever quantity label is being used in our shop, like details page,list page, modules, quick view, checkouts, orders view, invoices and order emails.
From here either we can use default from language or choose/write our custom quantity label.
Custom quantity on frontend
These settings can be overridden from category level and products level
The quantity label can be used as price/quantity (like $12/kg) by enabling "Price/Quantity Label in Detail Page/List page" configuration.
and it will show wherever price is displayed