  Fractional-order vector vortex beams are recently demonstrated to be new
carriers of fractional-strength optical vortices. However, why can those new
vortex beams formed by the combination of both unstable states propagate stably
in free space? Here, we solve this scientific problem by revealing the physical
essence of propagable fractional-strength optical vortices in free space.Three
new understandings regarding those peculiar vortex beams are therefore
proposed, namely Abbe diffraction limit, phase evolution of vortex beam, and
phase binary time vector property.For the first one, owing to Abbe diffraction
limit, the inherent polarization modes are intertwined together, thereby
maintaining the entire peculiar vortex beams in free space. For the second one,
we demonstrate the phase evolution of vortex beam, which is the physical reason
of polarization rotation of fractional-order VVBs. For the third one, the phase
is not merely a scalar attribute of light beam, but manifests a binary time
vector property. This work provides entirely different physical viewpoints on
the phase of vortex beam and Abbe diffraction limit, which may deepen our
knowledge on the behavior of light beam in classical optics.
