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Definition of gross income (Arizona state tax filing requirements)
Many states have a simple method for assessing income tax on nonresidents.  If you have $X income in State A where you claim nonresident status and  $Y income overall, then you owe State A a fraction (X/Y) of the income  tax that would have been due on $Y income had you been a resident of State A.  In other words, compute the state income tax on $Y as per State A rules, and send us (X/Y) of that amount.  If you are a resident of State B, then State B  will tax you on $Y but give you some credit for taxes paid to State A. Thus, you might be required to file a State A income tax return regardless of how small $X is.  As a practical matter, many commercial real-estate  investments are set up as limited partnerships in which most of the annual taxable income is a small amount of portfolio income (usually interest income that you report on Schedule B of Form 1040), and the annual  bottom line is lots of passive losses which the limited partners report (but do not get to deduct) on the Federal return.  As a result, State A is unlikely to come after you for the tax on, say, $100 of interest income each year because it will cost them more to go after you than they will  recover from you. But, when the real estate is sold, there will (hopefully) be a big capital gain, most of which  will be sheltered from Federal tax since the passive losses finally get  to be deducted.  At this point, State A is not only owed a lot of money (it knows nothing of your passive losses etc) but, after it processes the income tax return that you filed for that year, it will likely demand that you file income tax returns for previous years as well.