In a build.gradle, inside of task.register, I can find this
group 'build'
description 'Build the project for the R1 target and assemble a zip with the APK and the client library'
classifier variantNameCap
I've read that classifier should be relative to the archive. But what is exactly the concept of classifier in Gradle?
It's a concept taken from maven - see https://maven.apache.org/pom.html and search for classifier.
The classifier distinguishes different artifacts build from the same project.
A classifier could be something like fat or shaded where all dependencies are in a fat jar and/or shaded.
Classifiers like debug or source are quite common. The debug artifact will contain all debug information and the source artifact will contain the source files.