It's the form that Social Security wants you to use to request or change federal tax withholding on Social Security benefits.

Yes, I know, not many people get Social Security and even fewer want tax withheld, but maybe a complete software package should offer it anyway?

I usually advise clients to find some other way to pay their taxes because it has been difficult to start or stop withholding on Social Security, but this seems to be the solution for one of my clients. 

The options offered are 7%, 10%, 12%, and 22%.  You can't specify dollar amounts or "withholding allowances."  The same form is used for withholding on unemployment, but the only choice is 10%.