  Two types of relativistic jets are suggested to form near accreting black
holes: a potentially ultrarelativistic Poynting-dominated jet and a
Poynting-baryon jet. One source of jet matter is electron-positron pair
production, which is driven by neutrino annihilation in GRBs and photon
annihilation in AGN and x-ray binaries. GRB Poynting-dominated jets are also
loaded by electron-proton pairs by the collisional cascade of Fick-diffused
free neutrons. We show that, for the collapsar model, the neutrino-driven
enthalpy flux (classic fireball model) is probably dominated by the
Blandford-Znajek energy flux, which predicts a jet Lorentz factor of
$\Gamma\sim 100-1000$. We show that radiatively inefficient AGN, such as M87,
are synchrotron-cooling limited to $\Gamma\sim 2-10$. Radiatively efficient
x-ray binaries, such as GRS1915+105, are Compton-drag limited to $\Gamma
\lesssim 2$, but the jet may be destroyed by Compton drag. However, the
Poynting-baryon jet is a collimated outflow with $\Gamma \sim 1-3$. The jet
from radiatively efficient systems, such as microquasar GRS1915+105, may
instead be a Poynting-baryon jet that is only relativistic when the disk is
geometrically thick. In a companion paper, general relativistic hydromagnetic
simulations of black hole accretion with pair creation are used to simulate jet
formation in GRBs, AGN, and x-ray binaries.
