The Increase Screen Brightness and Decrease Screen Brightness actions increase or decrease the screen brightness by one unit (one unit is 1/16th of the total range).
The Small Increase Screen Brightness and Small Decrease Screen Brightness actions (v9.0+) increase or decrease the screen brightness by one quarter of a unit.
When you use these actions the system volume overlay briefly appears on the screen.
These actions are the same as pressing the hardware brightness buttons, with or without the option&shift key. They typically affect the primary screen only.