class_level_instance_and_class_variable_test1.rb

class A
  class << self
    def add(ele)
      @array ||= []
      @array << ele
    end

    def array
      @array ||= []
    end
  end
end

class B < A; end

B.add 'b1'
B.add 'b2'
B.add 'b3'
A.add 'a1'
A.add 'a2'
A.add 'a3'

p A.array #=> ['a1', 'a2', 'a3']
p B.array #=> ['b1', 'b2', 'b3']

class_level_instance_and_class_variable_test2.rb

class A
  class << self
    def add(ele)
      @@array ||= []
      @@array << ele
    end

    def array
      @@array ||= []
    end
  end
end

class B < A; end

B.add 'b1'
B.add 'b2'
B.add 'b3'
A.add 'a1'
A.add 'a2'
A.add 'a3'

p A.array #=> ['b1', 'b2', 'b3', 'a1', 'a2', 'a3']
p B.array #=> ['b1', 'b2', 'b3', 'a1', 'a2', 'a3']