Why some package-private classes are not obfuscated by Proguard?

Question

Working with an Android project in Android Studio 3.2, having enabled Proguard and some specific rules, I\'m not able to figure out the following:

a specific package

Answer 1

In "/build/outputs/mapping/release/" folder there are few files ("usage.txt", "seeds.txt", etc..) that contain the REASONS of why and which classes/variables/methods/etc.. are not-processed/not-shrinked/ot-obfuscated via ProGuard utilities.