Final answer:
The OPT Keypad +/- in the Edit window with the F8 (Grabber) tool active is used to adjust the audio levels of a selected region (Clip).
Step-by-step explanation:
In the Edit window, when the F8 (Grabber) tool is active and a region (Clip) is highlighted, what does the OPT Keypad +/- do?
When the F8 (Grabber) tool is active in the Edit window of a video editing software, the OPT Keypad +/- is used to adjust the audio levels of the selected region (Clip). The OPT Keypad + increases the audio level, while the OPT Keypad - decreases the audio level. This allows you to modify the volume of specific parts of a video clip without affecting the overall audio.
For example, if you have a video clip with background music, but you want to lower the volume of a specific section where someone is speaking, you can use the OPT Keypad - to reduce the audio level for that region only.