Dont go too hard on the phrases or terms. Signifier just signifies the state of affordance. Affordance is its inherent ability to communicate its function. Do understand that when it comes to UX, there are two aspects to an object – affordance and signifiers. If you can optimize both, you can better optimize the product and its ability to communicate its purpose and function without any further explanation to the user. Once you optimize both, you can ensure better user experience.