Significant Of Object With Parents / Child

Below mentioned classes, create object system accept in both way so what is it significant.

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ABC obj = new Test();

Test obj = new Test();
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--file ABC class

class ABC{   public void disp()   {   System.out.println("disp() method of parent class");   }  
public void abc()   {   System.out.println("abc() method of parent class");   }  }

--file Test class

class Test extends ABC{   public void disp(){   System.out.println("disp() method of Child class");   }  
public void xyz(){   System.out.println("xyz() method of Child class");   }  
public static void main( String args[]) {   //Parent class reference to child class object 
ABC obj = new Test();  obj.disp();  obj.abc();   }}