This integration provides a search provider for Mattermost messages, a dashboard widget showing your latest @mentions and lets you send files to a Mattermost channel directly in Nextcloud Files.
The account configuration happens in the "Connected accounts" user settings section. It requires to create a personal access token in your Mattermost settings.
A link to the "Connected accounts" user settings section will be displayed in the widget for users who didn't configure a Mattermost account.
There also is a "Connected accounts" admin settings section if you want to allow your Nextcloud users to use OAuth to authenticate to a specific Mattermost instance.