The near-IR line of Mn I at 15262.7 A has been recently proposed as a new
tool for diagnosing the magnetic field strength and the magnetic energy density associated with unresolved magnetic structures due to the enhanced sensitivity
of the Stokes I profile. In this paper, the behavior of the line, focusing on
the properties of the Stokes I profile, is analyzed in detail with the aid of
state-of-the-art magneto-hydrodynamical simulations of the solar surface
convection. The line is synthesized taking into account that the splitting and
strength of the Zeeman components have to be calculated under the framework of
the Paschen-Back theory via the numerical diagonalization of the total
Hamiltonian, including the hyperfine and the Zeeman contributions. The peak
ratio and separation of the blue and red lobes of the emergent Stokes I profile
are shown to be correlated with the magnetic field strength when no smearing is
taken into account, while the correlation slightly degrades when diffraction and
stray-light contamination is accounted for. We also analyze the dependence of
the first two line moments with the magnetic field, showing that the first and
second moments can be used to trace the velocity and the magnetic field
strength, respectively. This correlation is still maintained for ground-based
observations.