FontMetrics.max_box

You're seeing just the function max_box, go back to FontMetrics module for more information.
Link to this function

max_box(pixels, font_metrics)

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Specs

max_box(pixels :: number(), metrics :: t()) ::
  {x_min :: number(), y_min :: number(), x_max :: number(), y_max :: number()}

Return a box that would hold the largest character in the font.

The response is scaled to the pixel size.

returns {x_min, y_min, x_max, y_max}