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).

It's the theoretical breakeven event (Q=1) when you use high power lasers rather than the magnetic confinement of a tokamak. The NIF is doing research on this inertial confinement fusion, where they shoot a lot of power with lasers on a tiny pellet with hydrogen. So you get extreme densities and temperatures, but not a lot of confinement time, i.e. the ions cool down _really_ fast. That shifts the curve a little bit.

In my opinion, it is a bit misleading to add this to the plot, because for this laser-based fusion, the theoretical breakeven lies really far from the practical, commercial breakeven. But as shown there on the gif, the NIF did manage to cross the Q=1 for inertial confinement fusion last year!