Continuing to develop extrospective awareness and sensitivity to the felt sense of metta leads to a perception of what’s referred to as the ‘energy body’, or ‘subtle body’.
This is essentially composed of any physically felt sensations with non-physical causes (i.e. not pressure, temperature, texture, etc.). The experience of the energy body is often described subjectively as a kind of “second body”, or separate layer of sensations, beneath the coarser and more obvious physical ones.