How do I determine whether to change Acceleration in the Specific Game Settings window or in the Global Device Settings window?

Changes made in the Global Device Settings Window affect all profiles. Changes in the Specific Game Settings window only affect the Profile that is active when you make the change.

What Effect does the Slider Position have on Mouse Movement?

When in Relative or Absolute with Acceleration Mode, the farther the slider is to the right, the farther the mouse will go proportionally as the speed with which you move it increases.