By default, when setting up your campaign, commissions are set to trigger off of any product. Linking a product allows you to trigger referral commissions off of one of your internal product IDs rather than Campaign ID.
Why should I link to a product ID?
If you only want to award a commission on the purchase of a particular product, we suggest linking your product ID. This will ensure that commissions are only created from this campaign when the specific product is purchased.
How do I add a product ID?
You can add a product ID to your campaign under the advanced options of the "Campaign Setup" section. For step-by-step instructions, check out this support article: How do I create a campaign?