> ## Documentation Index
> Fetch the complete documentation index at: https://docs.stigg.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Overview

**Add-ons** represent separately billable functionality that extends specific [paid](../../modeling-your-pricing-in-stigg/plans#paid-plans) or [custom](../../modeling-your-pricing-in-stigg/plans#custom-plans) plans. Unlike plans, customers can purchase multiple units of a given add-on.

Add-ons are tied to the lifecycle of the plan they extend. In other words, when a subscription expires, the customer also loses access to the plan’s associated add-ons.

The functionality included in each add-on is defined by a combination of features and their configurations, collectively referred to as [**entitlements**](../../managing-customers-and-subscriptions/entitlements/overview).

Because add-ons extend the functionality of plans, when a subscription includes both plans and add-ons that define the same configuration or metered features, the total entitlement for each feature is the **sum** of the respective entitlements.

For example, if a customer’s plan includes **X** units of a specific feature, and they purchase an add-on that provides **Y** additional units of the same feature, the customer will be entitled to **X + Y** units in total. Moreover, if the customer purchases a quantity of **Q** add-ons, they’ll be entitled to **X + (Y × Q)** units of that feature.