Are healthcare coverage supplements based on total income or taxable income?

It is based on Household Income, which is a term specific to ACA.

To be eligible for the premium tax credit, your household income must be at least 100 – but no more than 400 – percent of the federal poverty line for your family size, although there are two exceptions for individuals with household income below 100 percent of the applicable federal poverty line. Remember that simply meeting the income requirements does not mean you’re eligible for the premium tax credit. You must also meet the other eligibility criteria. 

For information about the two exceptions for individuals with household income below 100 percent of the federal poverty line, see the instructions to Form 8962.

Here are four things to remember about how your income affects your premium tax credit:

• The amount of the premium tax credit is based on a sliding scale, with greater credit amounts available to those with lower incomes. 
• If the advance credit payments made on your behalf are more than the allowed premium tax credit, you will have to repay some or all the excess. If your household income is 400 percent or more of the federal poverty line for your family size, you will have to repay all of your excess advance credit payments. 
• If your projected household income is close to the 400 percent upper limit, be sure to carefully consider the amount of advance credit payments you choose to have paid on your behalf. 
• If your household income on your tax return is more than 400 percent of the federal poverty line for your family size, you are not allowed a premium tax credit and will have to repay all of the advance credit payments made on behalf of you and your tax family members.   

For purposes of claiming the premium tax credit for 2017, the following table outlines household income that is at least 100 percent but no more than 400 percent of the federal poverty line. 

For purposes of the premium tax credit, your household income is your modified adjusted gross income plus that of every other member of your family (see question 6) who is required to file a federal income tax return. Modified adjusted gross income is the adjusted gross income on your federal income tax return plus any excluded foreign income, nontaxable Social Security benefits (including tier 1 railroad retirement benefits), and tax-exempt interest received or accrued during the taxable year. It does not include Supplemental Security Income (SSI).

https://www.irs.gov/affordable-care-act/individuals-and-families/questions-and-answers-on-the-premiu...