public BuildTarget selectedBuildTarget { get; }

This property returns the current build target platform (similar to EditorUserBuildSettings.activeBuildTarget), however this property is provided specifically as a way for you to create a dependency on the build target within a scripted importer.

You should use this property if you are writing a scripted importer and you want your importer to generate different results based on the target platform. Using this property within a scripted importer creates a platform dependency in your importer. This means when you change the Editor's active build target, all assets imported by this importer will be automatically re-imported to reflect any changes caused by the dependency you set up.

Note: If you are not writing an importer, and just want to read the currently selected build target, you should use EditorUserBuildSettings.activeBuildTarget.