  We investigate the nature of the new cross term for Gaussian
parameterizations of Bose-Einstein correlations of identical particles emitted
from purely chaotic hadron sources formed by relativistic heavy ion collisions.
We find that this additional parameter in the so-called Bertsch
parameterization can be expressed in terms of a linear ``out-longitudinal''
correlation coefficient for emission of bosons and two already known ``radius''
parameters, $R_l$ and $R_o$. The linear correlation coefficient is of
kinematical nature and can be used to determine the widths of longitudinal
momentum distributions.
