A template part area is a taxonomy term

Declaring a part as a header or a footer is what makes it appear in the right place in the editor, and the mechanism underneath is a taxonomy.

{
  "templateParts": [
    { "name": "header", "title": "Header", "area": "header" },
    { "name": "footer", "title": "Footer", "area": "footer" },
    { "name": "sidebar", "title": "Sidebar", "area": "uncategorized" }
  ]
}

// the areas are header, footer and uncategorized. that is
// the complete list in 2022.

Only three areas exist, so anything that is not a header or a footer is uncategorised — which affects where it appears in the editor and the HTML element it is wrapped in. The area also determines the semantic tag, so a part in the header area renders inside <header> without the template saying so.