  Detailed analysis of the measurements of differential cross sections in the
forward region of pp elastic scattering at 13 TeV in LHC is performed. The
structures of the real and imaginary parts of the scattering amplitude are
investigated, both requiring exponential and linear factors. Representations of
the data in different conditions are compared, accounting for predictions from
dispersion relations and investigating the role of the Coulomb-nuclear
interference. The additivity of nuclear and Coulomb eikonal phases to determine
the interference phase in pp scattering is submitted to a comparison with
assumption of phase equal to zero. The structures of the real part of the
scattering amplitude under the two assumptions are examined, and direct
comparison is performed with the information extracted from the data. The
alternative criteria for the treatment of the interference phase lead to
different values for the $\rho$ parameter between 0.1 and 0.13 while $\sigma$
differ by 0.18 mb, and it is shown that the available data cannot discriminate
the two models.
