This tag can be used by app stores such as kde discover, gnome software, elementary appcenter, flathub or similar to discern age-appropriate content.
It uses the Open Age Ratings Service (OARS) in order to provide the required information.
This file was originally written by Richard Hughes, for Fedora, who
proposed it on the Transmission forum here:
https://forum.transmissionbt.com/viewtopic.php?f=3&t=16443
It's used to display a nice description and screenshot in GNOME
Software. The format is documented here:
https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html
The <_name/> and <_summary/> fields match the Name= and Comment= fields
in the .desktop file so are already translated.
The screenshot is taken with Transmission 2.92 (14714) on Fedora 25.
It's at one of the sizes recommended by the GNOME Shell Screenshot
Window Sizer extension.
The <update_contact/> is based on the Git history.
Charles Kerr