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## Resource: Introduction

## Article: Introduction

## Article Link: https://dev.wix.com/docs/api-reference/app-management/app-instance/introduction.md

## Article Content:

# About the App Instances API

The App Instances API allows you to collect data about the Wix sites that have installed your app 
and when a subscription for your app is modified.

With the App Instances API, you can:

+ Get notified by Wix when a Wix user installs your app on their site.
+ Track data that's associated with individual app instances. For example, to check whether the Wix user has installed a free or paid version of your app, 
    or to check what apps made by Wix are installed on the site.
+ Receive notifications from Wix when Wix users modify or cancel their subscription for your app.

See the [Billing API](https://dev.wix.com/docs/rest/app-management/app-billing/billing/introduction.md)
for more information how to charge Wix users for using your app.

## Before you begin

It’s important to note the following points before starting to code:

+ You must [authenticate as a Wix App](https://dev.wix.com/docs/rest/articles/getting-started/about-identities.md#wix-app).
+ Wix users can install only a single version of your app on their site. For
  example, they can't install a free and paid version of your app at the same
  time.

## Use cases

+ [Reach out to new users](https://dev.wix.com/docs/api-reference/app-management/app-instance/sample-flows.md#reach-out-to-new-users)
+ [Identify a site's installed apps made by Wix](https://dev.wix.com/docs/api-reference/app-management/app-instance/sample-flows.md#identify-a-sites-installed-apps-made-by-wix)

## Terminology

+ __App instance__: Specific occurrence of your app on a particular Wix site.
+ __Paid plan__: A version of your app that isn't free. This version could require a single one-time payment or a
  subscription based payment.

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