  Relativistic jets are observed only in the low/hard and intermediate states
of X-ray binaries (XRBs), and are switched off in the thermal state, but they
appear to be present in both low-luminosity and luminous active galactic nuclei
(AGNs). It is widely believed that strong large-scale magnetic fields is a
crucial ingredient in jet production; such fields can be attained only through
efficient advection from the outer disc. We suggest that geometrically thin
accretion discs with magnetic outflows are present in luminous radio-loud AGNs;
this is likely because the interstellar medium provides both mass and
sufficient magnetic flux to the outer disc. Most angular momentum of such disc
is removed by the outflows, and the radial velocity of the disc is
significantly increased compared to viscous drift velocity. This facilitates
efficient magnetic field advection through the disc to produce a strong field
near the black hole in luminous AGNs, which helps launch relativistic jets. In
XRBs, the magnetic fields of the gas from companion stars are too weak to drive
outflows from outer discs. Jets are therefore switched off in the thermal state
due to inefficient magnetic field advection in the disc.
