Android has a level of self protection from rogue apps and any app not installed via the play store is considered a potentially rogue app (by google). Android OS won't allow the installation of apps from anywhere other than play, without a little bit of work by the user. This is when you need to sideload.
The DAB+ app is often factory installed to your android headunit but through some mistake it has been deleted or you initiated a factory rest, the DAB+ app in no longer present. Or maybe you tried the DAB-Z app and removed the DAB+ app. Now you want the DAB+ app back and its just not there.
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