The BSP protocol defines an sbt extension.

It specifies an SbtBuildTarget with sbt-specific information. This is used by IntelliJ to provide the correct highlighting and completions for a given sbt version.

Ideally, a project's meta-project will be returned in a workspace/buildTargets response recursively, though for practical purposes one level is usually enough and the current state of the art in the IntelliJ Scala plugin.