What is the shaded black area on cardio load graph?

[Eve6]

What is the black area used for on the cardio load graph. I keep being in this area and just to the left of it? Is where I should be aiming to be at?

[alexthecat · Accepted solution]

@Eve6 it depends on what your fitness goals are. Here is my Cardio Load graph for today: 

 The light green segment on the far left is "undertraining". Moving to the right, there are two darker green segments that indicate "maintaining". Then two darker segments for "improving" and finally another light green segment for "overtraining". 

[Eve6]

Thanks . 
I am maintaining so that’s the aim though I do hit improvement too.

i searched everywhere for a graph explanation and no luck so thank you for your response.

[Eve6]