  $\Sigma$ is a new correlation measure, quite recently introduced to heavy-ion
physics. This measure, defined in the independent source model as a strongly
intensive quantity, is expected to be free of the effects of system volume and
volume fluctuations. This paper discusses the forward-backward correlation
quantified with the $\Sigma$ observable calculated in the framework of the
wounded nucleon model (WNM) and wounded quark model (WQM). Findings show that
the wounded constituent approach outperforms the commonly used heavy-ion Monte
Carlo generators, such as HIJING, AMPT or EPOS, by accurately describing the
experimental data on FB correlations with $\Sigma$ measured by ALICE in Xe--Xe
reactions at $\sqrt{s_{\rm{NN}}}$=5.44 TeV and in Pb--Pb collisions at
$\sqrt{s_{\rm{NN}}}$= 2.76 and 5.02 TeV. This paper demonstrates that $\Sigma$
can be a unique tool for determining the fragmentation function of a wounded
constituent in a symmetric nucleus-nucleus collision. However, in the wounded
constituent framework, it is no longer a strongly intensive quantity.
