This is the maximum to which a helium-filled core can grow, and if this limit is exceeded, as can only happen in massive stars, the core collapses, releasing energy that causes the outer layers of the star to expand to become a red giant.
This marks the maximum extent to which a helium-filled core can grow, and if exceeded, as is only possible in very massive stars, the core collapses, leading to an energy release that causes the outer layers of the star to expand and become a red giant.