The blog is referring to source- and binary-API compatibility guarantees that Spark provides.<p>IIRC, in order for DataFrames and Datasets to share a common parent class certain common methods&#x27; return types would need to change in a binary-incompatible way that would break DataFrame code which was compiled against older Spark versions.<p>Due to Spark&#x27;s API compatibility and versioning policies, this type of compatibility break can only occur in major releases, so it can only be done in Spark 2.0.0 and not Spark 1.6.