Sell-through is always expressed as a percentage. Net sales essentially refers to the same thing, in absolute numbers.
Sell through refers to sales made directly (Direct sales). Sell in, on the other hand, refers to sales made through a channel.
In the home video industry, sell-through refers to units which are sold directly to consumers, rather than to stores that rent them out. Sell-through priced videocassettes were generally under $30 in the United States, as contrasted with full-priced titles, generally priced at over $80 retail. With the coming of movies on DVD, sell-through pricing became the norm for DVDs in the United States.
Sell-through is a percentage of units sold during a period (usually 1 month).
It is calculated by dividing the number of units sold by the beginning on-hand inventory (for that same time period).
During the month of May you sell 100 units. You received 300 units in receipts. You end May with 900 units of stock (EOM, End of Month stock). What was your Beginning On-Hand units and what was your Sell-through?
Beginning of Month stock (BOM) = EOM 900 units - Receipts 300 units + Sales 100 units = 700 units
Sell-through = Sales 100 units / Beginning Inventory (BOM) 700 = 14.3% Sell-through in May.