Standard Enthalpy of Formation in Most Stable Form

[Shirley Liu 2I]

In lecture notes, it says "by definition, the standard enthalpy of formation of an element in its most stable form is zero" and an example is given: O2(g) -> O2(g). What does this mean? Wouldn't an element in its most stable form not undergo any changes so there wouldn't need to even have delta Hrxn anyways?

[annabel 2A]

I think it was supposed to be a really redundant obvious point... so I think you're right