Hello Vishnu, how are you guy?
Well, about your question, follow below a explain:
The available source types depend on the selected Heat conduction in solids option:
if Heat conduction in solids is disabled
Heat Transfer Rate (W) or Heat Flux (W/m2)
The heat flow from solid to fluid is governed by the specified Heat Transfer Rate (Heat Flux) value.
The specified Heat Transfer Rate is distributed among the selected faces in proportion to faces areas.
if Heat conduction in solids is enabled
Heat Generation Rate (W) or Surface Heat Generation Rate (W/m2)
The heat flow from the fluid to the solid is governed by the surface temperature which is a function of the specified Heat Generation Rate (Surface Heat Generation Rate) and heat conduction in the solid. In other words, the surface temperature changes depending on the intensity of heat conduction in the solid near the surface and the specified value of the Heat Generation Rate (Surface Heat Generation Rate), resulting in changes in heat flow.
The specified Heat Generation Rate is distributed among the selected faces in proportion to faces areas.
Basic, the difference there is because in Flow the temperature depend of fluid and SW Simulation depend of solid body.
Have a nice day man.