Github user dszeto commented on the issue:
https://github.com/apache/incubator-predictionio/issues/282
The short answer is no. It is totally arbitrary and depends on what your
engine accepts. Take recommendation engine template as an example:
https://github.com/PredictionIO/template-scala-parallel-recommendation/blob/develop/src/main/scala/DataSource.scala#L30
This engine template's data source reads events with `entityType` of
`users`.
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