This solubility curve represents equilibrium, and many crystallization steps are much too fast for attaining equilibrium. The line between the yellow and magenta regions on the same figure is not exact and depends on several factors, one of which is cleanliness. Tiny dirt can provide sites for nucleation. Appreciating that the change from one zone to the other is an approximation, we can see three distinct regions on the graph. The lower region corresponds to permutations of temperatures and concentrations where the material is soluble. If crystals were added, they would dissolve.
The next region is labled "supersaturated"; this is also called the metastable region. Although above the saturation line,
there is no certainty that crystals will form. There are cases where solutions have
been held for many years with no crystallization. When seed crystals are added or when
nucleation is induced by dust particles or by scratching the walls of the
container, crystals can form quite rapidly. The further above the saturation line, the
more likely that crystals will form because the driving force is greater.
Same ideas worded slightly differently.
Above the approximate boundary and into the yellow region, it is very difficult to avoid spontaneous crystallization. The metastable region is of greatest interest because that is where conditions can be adjusted to get the desired crystal properties.