The GeoPackage Related Tables Extension (RTE) defines the rules and requirements for creating relationships in a GeoPackage data store between geospatial or attributes data tables and other tables that contain or reference related content, such as attributes or media. One use case for this extension is to associate features with related multimedia content such as photographs, audio or video files, and/or PDFs.
There are many examples of where the RTE can be used, including relating parcel (land lot) features to pictures of that parcel, or linking census boundaries to the related demographic census data.
This extension, like all GeoPackage extensions, is intended to be transparent and to not interfere with GPKG compliant, but non-supporting, software packages. The OGC GeoPackage (GPKG) Related Tables Extension Interoperability Experiment verified that the extension is correctly designed to meet the design goals and to be transparent in this manner. The goal of the IE was achieved by building GeoPackages containing embedded multimedia content and sharing those GeoPackages with other software products. This IE produced an OGC Engineering Report (published soon) that discusses whether the extension is fit for use and adoption by the OGC.