The Nodal Source only supports the Nodal target, but geometric entities (points, curves, surfaces, solids, etc.) can still be applied. The Nodal Source load is ultimately applied to nodes rather than elements. The nodal source is the heat, not heat flux (heat per area). The nodal source is applied on each node, regardless of the control area or volume of this node. That is to say, all the nodes for this load will have the same amount of heat source, even for those nodes at the corner or edge of a region, unless the spatial field is used.