The Medicare Part D Benefit Types as shown in PDP-Facts our Part D Plan Statistics (National and State) are defined by the Center for Medicare and Medicaid Services (CMS) as follows:
defined standard (DS) benefits, actuarially equivalent (AE) standard benefits,
basic alternative (BA) benefits, and
enhanced alternative (EA) coverage
"These terms were intended to provide explicit guidance on permissible benefit design parameters for plan sponsors and actuaries. The first three benefit types are considered basic prescription drug coverage, and are actuarially equivalent to the defined standard benefit established in statute. These basic benefit designs vary only in terms of whether cost sharing tiers are applied versus one level of coinsurance, the deductible is lowered or eliminated, and the initial coverage limit is increased. "
The information paraphrased below is from Chapter 5 of CMS Publication 100-18 Medicare Prescription Drug Benefit Manual which gives more details regarding the benefit types
Defined standard (DS) benefits: has an annual deductible, has 25% coinsurance in the initial coverage phase, beneficiary is 100% responsible for costs in coverage gap.
Actuarially equivalent (AE) standard benefits: has an annual deductible, the plan may substitute certain cost-sharing requirements in defined standard coverage including tiered structures tied to plan formularies or preferred pharmacies in a plan's network.
Basic alternative (BA) benefits: may have a reduced or $0 deductible, can use tiered copayments or coinsurance, may have a modification to the initial coverage limit. Remains actuarially equivalent to the standard benefit.
Enhanced alternative (EA) coverage: a plan whose value exceeds that of the defined standard coverage. It includes the basic prescription drug coverage and has supplemental benefits which may include: a reduction in cost-sharing in the "coverage gap", a reduction in or elimination of the initial deductible, a reduction in the coinsurance or copayments applicable during the initial coverage phase, an increase in the initial coverage limit, and/or supplemental drugs.