I found the answer of what type of income it is looking for on a different guidance page:
"What if I don't have Form 6251?
...
If you think your AMTI might be different than your "regular" taxable income (Form 1040, line 10), you'll need to determine what your 2019 Alternative Minimum Taxable Income was and enter it here. If you don't know your AMTI, most people will get the same result using your "regular" taxable income (2019 Form 1040, 10)."
So it is looking for taxable income. But this guidance page also references the wrong 1040 line number for 2019. Line 10 was "taxable income" in 2018 form 1040.
