Why Spock Framework recommends to use SUS, but there is no such abbreviation in any example?

Spock framework states that we should use SUS abbreviation for testing entity, but never uses it itself.

Here is the morsel from documentation: Spock Terminology

Here are Spock official examples from GitHub

What is the right one?

For example:

class EmptyStackSpec extends Specification {
  def stack = new Stack()

  def "size"() {
    expect: stack.size() == 0
  }
}

OR

class EmptyStackSpec extends Specification {
  def sus = new Stack()

  def "size"() {
    expect: sus.size() == 0
  }
}

So, I'm puzzled. Why does it present in docs if nobody uses it?