In simplest terms

AFWall(Free) + Unlocker = AFWall (Donate)

This is a unlocker for AFWall+ (Free) version to use donate features. You need to have AFWall+ (Free) version to use it with unlocker

AFWall+ Free - Free version of AFWall+
AFWall+ Donate - Donate version of AFWall+
AFWall+ Unlocker - Unlock donate features in AFWall+ Free version

Why I should buy AFWall+ Unlocker compare to AFWall+ Donate ?

1. Test AFWall+ Beta versions (usually updated first in AFWall+ Free version)
2. Supporting development by IAP
3. Works on multiple devices