It's not nebulous at all, it's incredibly fixed by law.
3 years (from the later of date of filing or the due date) to audit any return, for any reason. The 3 year statute of limitations applies to taxpayers seeking refunds by filing an amended return. Note that because the amended return is essentially a new return, the IRS gets 3 years to audit the amended return.
6 years (from the later of date of filing or the due date) to audit a return with a substantial undereporting of gross income or overstatement of deductions in credits resulting in a 25% or more understatement of taxable income
No deadline for returns that were not filed. Because obviously you can't audit a return that hasn't been filed.
There is also no deadline for fraudulent returns. Fraud is something more than the type of things that would trigger a 6-year audit window, like trying to avoid tax entirely, or taking advantage of a deduction or credit for which it's clear that the taxpayer wouldn't qualify for without some active effort to falsify their return. (Think Wesley Snipes.)
3 years (from the later of date of filing or the due date) to audit any return, for any reason. The 3 year statute of limitations applies to taxpayers seeking refunds by filing an amended return. Note that because the amended return is essentially a new return, the IRS gets 3 years to audit the amended return.
6 years (from the later of date of filing or the due date) to audit a return with a substantial undereporting of gross income or overstatement of deductions in credits resulting in a 25% or more understatement of taxable income
No deadline for returns that were not filed. Because obviously you can't audit a return that hasn't been filed.
There is also no deadline for fraudulent returns. Fraud is something more than the type of things that would trigger a 6-year audit window, like trying to avoid tax entirely, or taking advantage of a deduction or credit for which it's clear that the taxpayer wouldn't qualify for without some active effort to falsify their return. (Think Wesley Snipes.)