TDD/IOC Help

gistfile1.cs

public interface ISomething
{
  void DoSomething();
}

public class SomethingA : ISomething
{
  public void DoSomething()
  {

  }
}

public class SomethingAB : ISomething
{
  public void DoSomething()
  {

  }
}

public class Different
{
  private ISomething ThisSomething;
  
  public Different(ISomething Something)
  {  
    ThisSomething = Something;
  }
}

//NOTES: In some circumstances in your app you may want SomethingA to be used in class Different and sometimes SomethingAB. How do you tell the IOC when to use the relevant class?