Use this property to reference the output folders or build report directories from one or more BuildPipeline.BuildContentDirectory builds.
The types used in those builds are included in the information provided to UnityLinker.
This ensures that the Player can load all the content from those additional builds.
During a build you can also add directories from a build callback with BuildPlayerContext.AddPreviousBuildReportDirectory.
Additional resources: BuildPipeline.BuildContentDirectory, BuildPlayerContext.AddPreviousBuildReportDirectory