  We demonstrate that the present superaccurate measurements of transition
processes between atomic states in hydrogen atom reached the limit of accuracy
when transition frequency cannot be defined anymore in a unique way. This was
predicted earlier and is due to the necessity to include the nonresonant
corrections in the description of resonant processes. The observed spectral
line profile becomes asymmetric, and it becomes impossible to extract the value
of transition frequency from this profile in a unique way. Nonresonant
corrections depend on the type of experiment and on the experimental
arrangement. However, the line profile itself for any resonant process can be
defined with any desired level of accuracy. A popular trend in modern search
for atomic frequency standards and atomic clocks is the search for transitions
where the nonresonant corrections are negligible. In this paper we present
closed expressions for the resonant photon scattering cross sections on an
atomic level with dependence on all atomic quantum numbers including fine and
hyperfine structure. These expressions are given for different types of
experiments with fixing of the incident (outgoing) photon propagation
directions and incident (outgoing) photon polarization. Using these expressions
we demonstrate that the transition frequencies in particular cases cannot be
derived uniquely if the accuracy of measurement reaches the level quoted in A.
Beyer {\it et al}. Our interpretation of the results of this experiment is
alternative to the interpretation given by A. Beyer {\it et al}.
