What is the difference between proguard-android.txt and proguard-rules.pro ? – Android

The getDefaultProguardFile('proguard-android.txt') method obtains
the default ProGuard settings from the Android SDK tools/proguard/
folder. The proguard-android-optimize.txt file is also available in
this Android SDK folder with the same rules but with optimizations
enabled. ProGuard optimizations perform analysis at the bytecode
level, inside and across methods to help make your app smaller and run
faster. Android Studio adds the proguard-rules.pro file at the root
of the module, so you can also easily add custom ProGuard rules
specific to the current module.

Please refer this: https://developer.android.com/studio/build/shrink-code

Meaning that you should add your custom proguard file into proguard-rules.pro,if you want to separate some rules to many files,you can do it and declare them after this:

proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'