The tax code does not use the term "MAGI" or "modified AGI," so Congress has nothing to do with this. For a particular purpose, the tax code simply specifies that actual method. § 219 is the section of the tax code that covers the deduction for a traditional IRA contribution. For the purpose of the phaseout of this deduction, § 219(g)(3)(A) says:
(A)Adjusted gross income Adjusted gross income of any taxpayer shall be determined—
(i)after application of sections 86 and 469, and
(ii)without regard to sections 135, 137, 221, 222, and 911 or the deduction allowable under this section.
For any particular purpose, the modifications to AGI depend on the particular purpose, so there can be no one set of modifications, contrary to what is implied by the misleading TurboTax article.
