the region "D" is the target range.
C works okay too, but has much less wiggle room, you want to dose more as you start hitting that concentration.
CO2 and light also have similar curves.
For nutrients, we do not know what the E ranges are for plants, mostly salt stress are the cases that are known. Hoagland's solution has anywhere between 210-235ppm of N for example, 50ppm or thereabouts for PO4 etc.
This is often used as a non limit reference for growth studies.
So it's very safe to say plants can handle a wide upper range beyond most even the more serious mistakes folks often make without risk.
Still, hitting the concentration range of D is far easier than say B or C which are far narrower/much harder to maintain from a management perspective.
Also, if you strongly limit a nutrient, say PO4, then the CO2 demand also goes down because the PO4 is more limiting than say CO2 might be.
This dependence leads some to think there's more to nutrients at lower limiting levels than is really the case. When the nutrients are independent, then demand goes up. Likewise, if the light is more intense, then the demand for CO2 and nutrients also goes up. But if you do not add more CO2/nutrients, then such light is wasted and cannot be used for growth.
Likewise, the non limiting nutrients allow the light and CO2 to be fully used.
If you do not have good CO2, then the light is also not fully used at peak efficiency.
So a combo of low light, good CO2 and good nutrients along with sediment nutrients gives the best overall management for most aquarist' plant growing goals.