Nextcloud Deck is a kanban-style app for planning and project management inside Nextcloud, comparable to Trello. You create cards and lists, organise them with labels, write notes in Markdown, attach files, discuss work in comments and follow changes in an activity stream. Boards can be shared with colleagues, including through the Circles app. Deck does not run on its own: it is an app you install on a Nextcloud server, from the Nextcloud App Store or the release page.
Deck is released under the AGPL-3.0 licence. Nextcloud is developed by Nextcloud GmbH, which has offices in Stuttgart and Berlin, Germany. Because Deck runs inside Nextcloud, the administrator of that Nextcloud environment decides where the data lives: on your own servers or with a hosting provider of your choice. Nextcloud itself states that Nextcloud GmbH has no access to customer data in self-hosted installations. For a European organisation that already uses Nextcloud, Deck therefore adds no additional external service or vendor.