Mandatory Backup Withholding
If you are subject to backup withholding then before you submit the form you have to look to the Certification section and cross out the second section of three lines that starts with:
2. I am not subject to backup withholding .....
You really should read the form's instructions in the areas that you are working on. This will help you to be accurate in filling it out.
I have filled out many W9 forms and learned that after you fill out one or two it becomes remarkably simple the following times.