This picture is from the paper of F. J. Narcowich "Narcowich, F.J., 1980. Analytic properties of the boundary of the numerical range. Indiana University Mathematics Journal, 29(1), pp.67-77."
I understood the proof of Theorem 3.3 except the last portion " with an appropriate labeling...". I have gone through the book of Functional analysis by B. Sz.-Nagy (Page: 376) but unable to relate this point.
Kindly if someone give an idea to proceed.