The download icon downloads:

• templates for an Inkscape plugin from the “pattern definition” panel
• the diagram as scalable image from the other panels

Templates

A template is a text file, please preview before trying to apply: it might contain an error message. The plugin renders regular or circular patterns from the download file. These templates are not available on iOS nor Android (read: tablets, smartphones), Inkscape itself does not run on these systems.

Diagrams

The download icon for diagrams launches a download dialog on a mouse click, when tapping, you get a link that opens the dialog. You can view a download in a browser by dragging it with your mouse between the tabs:

Some bowsers may have a limit on the size of the downloads.

Some tablets open a download in a browser, some of those fail to render. Other save a download in a photo section, but it is a vector based image, not a pixel based photo. A 3rd party editor supporting SVG can show and change the download. It may need to get access to the photos section or wherever the download is stored.

A use case to edit thread diagrams: apply other colors/widths to the threads.