The Cybersecurity and Infrastructure Security Agency (CISA) has developed—in collaboration with other federal agencies, state/local governments, and the private sector—an “Essential Critical Infrastructure Workforce” advisory list. This list is advisory in nature. It is not, nor should it be considered, a federal directive or standard.
Here’s a link to their guidance.
In addition, Curi has developed a letter template for medical workers in jurisdictions with stay-at-home orders to use as documentation when traveling to and from work. The template is available here.