  We study the origin of the global $B-L$ symmetry in a class of flavor-unified
theories with gauge groups of ${\rm SU}(N\geq 6)$. In particular, we focus on
the ${\rm SU}(8)$ theory which can minimally embed three-generational SM
fermions non-trivially. A reformulation of the third law for the flavor sector
proposed by Georgi is useful to manifest the underlying global symmetries. The
't Hooft anomaly matching and the generalized neutrality conditions for Higgs
fields play the key roles in defining the $B-L$ symmetry. Based on the global
$B-L$ symmetry, we count the Higgs fields that can develop the VEVs and the
massless sterile neutrinos in the ${\rm SU}(8)$ theory. We also prove that a
global $B-L$ symmetry can always be defined in any ${\rm SU}(N\geq 6)$ theory
when it is spontaneously broken to the SM gauge symmetry.
