Sales units (or sell-in) are noted the moment a sales client makes a purchase order (or once it is officially paid). The retailers, distributers, wholesalers...those are the sales clients.
Shipped units are noted the moment a device is moved off the packaging line and into the supply chain for delivery and are no longer under the direct ownership of the supplier/producer.
Sell-through or sold-through are units sold to end consumers. That's us.
The lagging indicator of sell-through from units sold or units shipped is typically weeks as determined by the delivery methodology, warehousing methods and shelf life.
Suggestion: As a sales tracking website, why not have a section on industry definitions? Or have tools that can simulate the difference between shipped and sell-through?