[sam]

Author here. The black curve represents the hot-spot ignition condition for a laser inertial confinement fusion (ICF) experiment (like the NIF). This means that during the short period of inertial confinement, the self-heating exceeds all losses in the hot spot leading to an increase in temperature due to self heating. It only applies to the black 'x' points.

The Q_sci^MCF contours correspond to scientific energy gain (ratio of fusion power to heating power crossing the vacuum vessel boundary) for a magnetic confinement experiment.

For ICF we can't draw simillar Q_sci^ICF contours because the total fusion energy released depends on the degree to which the ignited hot-spot propagates a burn in the surrounding cold fuel. And this depends on other variables like the symmetry of the implosion which are not captured in this plot.

If you're curious to read more about this check out Section III.F of the linked paper (pp.10-11).