Amount of details (%)
Determines how many details are visible in the resulting image. Influences the quantization tables directly.
The graph shows all 64 values of a quantization table lined up in a row, following the zigzag pattern of the JPEG specification.
On the X-axis, the first value therefore corresponds to the DC of the quantization table, followed by the AC values for the details of the image.
The gray line shows the values for a calculated quantization table (based on Appendix I of the JPEG specification) of a selected quality value, while the blue line shows the values of the actually used quantization table, which differs from the gray line if the "Amount of Details" slider was used.
In this case, the quantization values for the detailed areas of the image are larger and thus the image is less detailed.
To make it easier to see that larger values lead to a decrease in quality, the Y-axis has been reversed.
Practically this means: If you want to create the smallest possible image with good quality, first decrease the quality so far that you don't see any block artifacts anymore.
Block artifacts are the most obvious and disturbing artifacts, so they should be avoided in the first place.
Then reduce the amount of details as much as you find it acceptable.