This model was introduced by Altschuler and Newkirk (Solar Physics 9, 131, 1969) and by Schatten, Wilcox, and Ness (Solar Physics 6, 442, 1969). It replaces the MHD effects of electric currents in the body of the corona by fictitious currents at a fixed "source surface", now almost universally taken to be a sphere at a radial distance of 2.5 radii from Sun center. Any field line (as derived from a potential-field model based on photospheric measurements) that succeeds in reaching the source surface immediately bends to a radial direction and is "open". This convenient calculation works reasonably well, but one must have the following worries:
Probably an expert or a theorist could add to this list, or possibly provide a rebuttal to the criticism. They'd be welcome to write a Yohkoh science nugget on this topic, then!
ps. See the discussion in the paper by G. A. Gary, "Plasma beta above a solar active region: rethinking the paradigm," Solar Phys. 201, 71 (2001).