Are all applicable filing statuses are always explicitly spelled out in every IRS tax form, or are certain shorthands/assumptions made?
For example, the 2019 Form 8995 says it may be used to calculate a deduction, if (among other conditions):
[…] Your 2019 taxable income before your QBI deduction is less than or equal to $160,700 ($160,725 if married filing separately or a married nonresident alien; $321,400 if married filing jointly) […]
What income limit would apply if one's filing status were "Qualifying Widower"?
The simple interpretation here is that the limit by default is $160,700 unless the filing status is one of the two "married filing…" exceptions. That is, taking these instructions literally a "qualifying widower" [or "head of household"] is subject to the $160,700 phase-out threshold and the instructions for Form 8995-A only strengthen this (emphasis mine):
[…] if your taxable income (before the QBI deduction) exceeds the threshold ($160,725 if married filing separately or a married nonresident alien; $321,400 if married filing jointly; $160,700 for all others) […]
But my understanding is that "Qualifying Widower" is generally equivalent to "Married filing jointly" in most other contexts. The filing status instructions themselves say (emphasis again mine):
You can check the “Qualifying widow(er)” box at the top of Form 1040 or 1040-SR and use joint return tax rates for 2019 if all of the following apply. […]
So is this an overarching rule that says "Qualifying Widower" is treated as "Married filing jointly" for the rest of the return, or does each form/worksheet/line stand alone with its particular instructions re. filing status?