This page is obsolete. Redirecting to https://developer.android.com/studio/projects/android-library.html#aar-contents
The 'aar' bundle is the binary distribution of an Android Library Project.
The file extension is .aar, and the maven artifact type should be aar as well, but the file itself a simple zip file with the following entries:
These entries are directly at the root of the zip file.
The R.txt file is the output of aapt with --output-text-symbols.