C-, D, F-class neighborhoods

[Jim K.]

How do you draw the two dividing lines between these three types of neighborhoods? Is the A-D (and sometimes F) system in common use for grading neighborhoods the best possible system for those interested in investing in low-income, high-yield areas, especially in Rust Belt and East Coast cities?

Hoping this turns into a productive, informative thread that dives deeper into this topic than we usually seem to go here in the forum.