Consider a thermodynamic classical closed system. Due to the First and Second laws of thermodynamics, we know that $dU=\delta Q-\delta W=TdS-PdV$, and we could assign to $\delta Q=TdS$ the meaning of exchanged heat.
But what physical meaning could we assign to $SdT$ appearing, e.g., in the expression for a reversible Helmholtz free energy variation, $dA=-SdT-PdV$?