Fluid typography, and the clamp() you did not write

6.1 in November generates a clamp() from a font size, so type scales with the viewport without a stylesheet doing arithmetic.

{
  "settings": {
    "typography": {
      "fluid": true,
      "fontSizes": [{
        "slug": "large", "size": "1.5rem", "name": "Large",
        "fluid": { "min": "1.25rem", "max": "2rem" }
      }]
    }
  }
}

/* → font-size: clamp(1.25rem, 1.25rem + ((1vw - 0.2rem) * ...), 2rem) */

Turning it on globally applies a computed fluid range to every size that does not declare one, which is a site-wide typographic change delivered by a boolean. The generated formula is not something to read or override, so a theme with its own type scale should declare explicit min and max values per size rather than accepting the derived ones.