The Affordable Care Act includes a long list of codified instructions about what's required under the law. However, like any major piece of legislation, many of the specific details are left up to the agency responsible for implementing the law.
While the PPACA is itself a lengthy document, it would have to be several times longer yet in order to cover every conceivable detail involved in operating the ACA exchanges, Medicaid expansion and so forth. The major provisions of the ACA fall under the Department of Health & Human Services (HHS), and within that, the Centers for Medicare & Medicaid (CMS)
Every year, CMS issues a long, wonky document called the Notice of Benefit & Payment Parameters (NBPP) for the Affordable Care Act. This is basically a list of tweaks to some of the specifics of how the ACA is actually implemented.
If you look at the actual legislative text of the final version of the Patient Protection & Affordable Care Act (PPACA, or simply ACA), the table describing the applicable maximum percentage of income that exchange-based enrollees have to pay for their premiums looks like the table below:
(Notably missing is the lower-bound 100% FPL subsidy eligibility cut-off; there's a separate section of the law which notes the 100% threshold but makes an exception for certain lawfully-present immigrants who earn less than 100% FPL but who aren't eligible for Medicaid for various reasons and are given an exception).
