I believe what they are referring to as 'In-System Transfer', is the
'transfer orbits' required to move for the orbit of one planet to
the orbit of another. The 'transfer' orbit is the path taken by the
spacecraft. The high-energy transfer orbit has the star as one focus
of the ellipse, and the new orbit as the 'apocenter' of the ellipse.
(The apocenter is the part of the ellipse furthest from the star.)
The low-energy transfer orbit, or Hollman orbit, has the pericenter
at the starting orbit and the apocenter at the ending orbit.