The relationship between enthalpy change ($\Delta H$) and internal energy change ($\Delta U$) at constant pressure is given by the equation: $$ \Delta H = \Delta U + P\Delta V $$ Since the work done by the system at constant pressure is $\Delta W = P\Delta V$, the difference between $\Delta H$ and $\Delta U$ represents the work done: $$ \Delta H - \Delta U = \Delta W $$ Therefore, the difference indicates the work done.