Final answer:
A certificate of appealability must be obtained after a death penalty litigant's sentencing, specifically for federal habeas corpus appeals. It applies only to federal cases and is not required before trial or for all criminal matters. The Supreme Court continues to define the constitutional boundaries of the death penalty's application.
Step-by-step explanation:
A certificate of appealability is required to be obtained after sentencing and typically is an issue when a death penalty litigant is seeking to appeal a decision in habeas corpus proceedings. In the context of federal cases, a certificate of appealability is necessary when a defendant wants to appeal a federal court's final order in a habeas corpus proceeding. This rule is applicable exclusively to federal cases, notably after a habeas corpus filing has been denied, thus enabling the litigant to appeal to a higher court. It is not required before trial or simply because it involves a criminal matter. The Supreme Court plays a crucial role in determining the limits of the death penalty, ensuring that certain groups, such as those with cognitive disabilities or those who were minors at the time of their crimes, are not subjected to capital punishment.