Interesting Properties in Parallelism Show Plan Operator
From memory, the Predicate (and maybe ProbeColumn) is populated when the Parallelism operator (exchange) is the site of a bitmap filter application. Again, I might update this quick answer.
HashKeys (and maybe ProbeColumn) might exist in the showplanxml.xsd for aspects that are not normally exposed, or only applicable to certain technologies that share the schema e.g. possibly Azure/Hadoop. Apparently these attributes aren't used by PDW.