Hello once again BP Community!

Just a few questions about the ABCs of neighborhoods. I've heard a few guest on Bigger Pockets podcast refer to buying properties in B and C neighborhoods. Some say they stay clear of D neighborhoods often referencing safety and comfort as a concern. Essentially I've heard it said that if you aren't comfortable being in that neighborhood at night then that is a D or lower location. I also heard of the ABCs of neighborhoods referencing median income levels of residents in the area.

I mean no disrespect with this question if these letters represent a community in a negative light. This is not my intent. I'm just trying to get a better understanding. My question is this, Are the A,B,C,D and F rankings subjective to each investor? If this is not the case then where are people pulling this info? Are they just making it up as it applies to them and their investment interest? I would guess relating to income that A+/A is a upper class neighborhood with high median income levels. This would be like 6 figures. B would represent upper middle class, C middle class, D close to or at the poverty level and F I hear doesn't really exist. Or does it? 

I'm not here saying one is better than the other because each investor looks in every type of neighbor based on their own interest. Any light shed on this issue will be greatly appreciated!

Thank you,