Github user greghogan commented on the pull request:
https://github.com/apache/flink/pull/1778#issuecomment-221597445
`PARALLELISM_UNKNOWN` is a no-op which leaves the parallelism unchanged.
This is a useful default for batch algorithms such as `JaccardIndex` for which
parallelism can be optionally configured. If the user does not specify a
parallelism we do not want to override the parallelism of the
`ExecutionEnvironment` as would happen if we used `PARALLELISM_DEFAULT`.
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