Linking an ad specification to a product group means that the ad spec will only be listed for that particular product group.
You will be able to view the ad spec at the bottom of the product group page.
However, certain product groups can be linked to several media kits. If an ad spec is linked to a product group, and then that product group is linked to multiple media kits, the ad spec will be visible on all of those media kits as well.
If you prefer that your ad spec only appear on selected media kits, you can link the ad spec directly to your media kit instead of linking it to product groups.
If you choose to link your ad spec to your media kit it will appear under your media kit's Ad Specifications tab.