In addition to the Mattermost REST APIs, the Apps framework provides additional functionality:
Subscriptions - learn how to subscribe to notifications about Mattermost events. For example, have your App notified whenever a message is posted in a channel that mentions your
Key-value store - a simple key-value store for App-specific data.
App webhooks - learn how to subscribe to webhooks from third-party systems.
Mattermost API - learn how to use the Mattermost REST API client to interact with the Mattermost server.
Debug Apps - learn how to debug your App.