I think you refer to the imaginary part of P
P is one of the 3 secondary line coefficients
P = sqrt( (R+jwL)*(G+jwC) )
where R,L,G,C are the primary line characteristics measurable/length
Re(P) is the attenuation Coefficient
Im(P) is the wavelength Coefficient (2*pi/Lamdba) *****op's query***
Im(P) gives the distance along the line where the phase has changed by 2*pi)
and practically related to calculating line (eg coax) velocity factor.
Someone may be able to give current reference to the secondary coeficients. My book is too old.