Under regulations an employee may claim an additional withholding allowance or reductions in withholding for estimated itemised deductions and the estimated section 199A deduction (other than deductions for exemptions and above-the-line deductions), for estimated tax credits under chapter 1, and for additional deductions including the extra standard deduction available to older and blind taxpayers and whatever else the Secretary specifies. On the current Form W-4 these are the deductions and credits entries in steps 3 and 4(b), processed through Publication 15-T's computation.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.