COMPETENZE SOGGETTE A DETASSAZIONE
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US tax forms don't structure the terminology this way. Rather, "taxable income" is calculated by the texpayer using the pay stub data (or the data summarized annually on a Form W2, for example) as a starting point before factoring in exemptions, deductions, & adjustments most of which don't appear on the pay stub. The pay stub shows "gross pay" or "base pay" which is the total payment from which the employer subtracts various amounts before giving the employee the remaining net pay. I think soggette describes certain portions of gross pay that are subject to, or recognized in the calculation of, certain subtractions such as various taxes or other fees, etc., retained by the employer before paying out the net. These portions of gross pay overlap; e.g., portions are subject to both Medicare and income tax simultaneously but some are subject to income tax only. But instead of saying "amount subject to Medicare tax," e.g., the pay stub will say "Medicare amount," meaning portion of gross pay that is used in calculating Medicare tax. See Dr. Hagar's example here or this other cite here.