The Q Setting is a slider that allows you to determine the balance between quality and compression that you want for your image. It's best to start with the slider as far to the left as possible and slowly move it toward the right until you find an acceptable amount of quality. Most JPEG encoders use a set Q-Table when analyzing an image, which means they don't have the ability to adjust to different types of images. HVS JPEG comes with three Q-Table sets for broad groups of images. The General table gives reasonable results with most images.
Portraits is more dedicated to images with a lot of smooth tones.
This table emphasizes tonal transition and not detail. The Textured Images table should be used with images that have a lot of fine detail or texture that you wish to preserve. At the bottom of the list is the best choice of all, the Generate Optimized Q-Table button. This setting gives you the best quality for the most compression. However, it takes longer.