Please clarify: "A library component that is not built by gradle."

This is the description on PrebuildLibrary. Which of the following does it actually imply?

[1] Not built by Gradle via the native software model (can still be built within a Gradle project or task by other means)


[2] Already existing when Gradle starts (cannot be built via an external task or something like the GnuMake plugin within the same multi-project).