It is for personal income (business income is different.)

Well, strictly, adjusted gross income is income for purposes of taxing, but AGI is gross income after deductions allowed in the tax code, and "flat" tax proposals -- including Paul's -- almost without exception call for eliminating most deductions or all deductions (Paul expressly states that he will eliminate all except for charity and mortgage-related deductions.)

(Keeping one of the complexities of the current tax code, Paul continues to allow more deductions for businesses than individuals.)