An add-on provides additional functionality to the subscription that you purchase it for. It is not visible in the product catalog as a separate SKU, because it is linked to a parent offer and there is no possibility to purchase an add-on separately. Parent offer always comes first to be placed, and if there are add-ons to parent offer a new tab will display where a customer can select needed add-ons and proceed to the checkout.
Several offers available on the platform have optional add-ons that you can purchase for your subscription.
Add-ons are linked to a specific subscription. If you disable the subscription, the associated add-on is also disabled.
In more details read the article How do I add Add-ons when purchasing a new Subscription?