In Java we use enum to represent fixed set of constants

For example, we would define days of week enum type as follows

public enum Day {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
    THURSDAY, FRIDAY, SATURDAY 
}

In Scala, we can do the same thing by extending Enumeration, for example

object Day extends Enumeration {
  type Day = Value
  val SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY = Value
}

You can find examples of Scala Enumeration usage in Spark
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/TaskState.scala

https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/rdd/RDDCheckpointData.scala

https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/executor/InputMetrics.scala