Total Heat applies to all targets: Nodal, 1D, 2D, or 3D target. Total Heat is the total amount of heat to be evenly distributed over the entire application region. The Total Heat can be applied to 3D element faces, but not 3D elements. This is quite different from Total Volumetric Heat Generation, which can be applied to 3D elements, but not 3D element faces.

For the total volume heat option, a uniform distribution is automatically assumed, so a spatial field becomes irrelevant.

Total heat does not apply to volumetric heating. That is an option of Volumetric Generation.