PlantUML Diagrams for Confluence is an Add-on for Confluence (Cloud and Data Centre) that allows PlantUML and GraphViz DOT diagrams to be created, edited and stored on customers' Confluence instance. The Add-on only stores the text description and resulting image output on customers' Confluence instance.
Cloud based add-on sends the diagram markup through secured servers where preview image is generated. Both the markup and the resulting image are persisted only on the Confluence instance.
For on-prem based add-on, there is no outgoing traffic. Everything is contained within the server.
The PlantUML website has thorough documentation on how to build the various UML diagram types it supports.
The documentation mostly covers the paid version of the add-on.