Yes, it's not a placeholder, it's a https://www.scala-lang.org/api/2.12.10/scala/reflect/ClassTag.html thing, that is required here https://spark.apache.org/docs/latest/api/scala/index.html#org.apache.spark.rdd.RDD@map[U](f:T=%3EU)(implicitevidence$3:scala.reflect.ClassTag[U]):org.apache.spark.rdd.RDD[U]
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