The includes segment node is restricted to users who have an an array trait which includes a specific value. This can be useful for “tagging” users with a set of values, and then asserting that a user has at least one of those tags.

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Example Use Case - Messaging Users

Let’s say you have multiple user tags e.g. “VIP”, “Frequent Buyer”, “Loyalty Program Member”. You can issue an “identify” event with an array trait containing all of these tags. 
{
  "type": "identify",
  "userId": "1234",
  "traits": {
    "tags": ["VIP", "Frequent Buyer", "Loyalty Program Member"]
  }
}
You can then create a segment that includes users who have at least one of these tags.
Includes Segment Example