However, if you are digging through your old game collection or trying to install the game via Steam, you might run into confusion regarding product keys and retail codes. Because Opposing Fronts was a "standalone expansion," the key system works differently than most other games.
If you have a code that looks like XXXXX-XXXXX-XXXXX-XXXXX-XXXXX , you have the key. Whether the seller calls it "Retail" or "Product" is irrelevant; the activation process is the same.
by selecting "Activate a Product on Steam" from the Games menu. Retail Code
: If your product key is reported as invalid, double-check that you have entered it correctly. Make sure there are no typos and that you haven't exceeded the character limit.