code: static function getUnits()
      {
          return [
              static::PIECE,
              // Length
              static::METER,
              static::CENTIMETER,
              static::MILLIMETER,
              static::INCH,
              static::FOOT,
              // Weight
              static::KILOGRAM,
              static::GRAM,
              // Volume
              static::CUBIC_METER,
              static::LITER,
              static::MILLILITER,
              // Duration
              static::DAY,
              static::HOUR,
              static::MINUTE,
              static::SECOND,
          ];
      }
docs: Returns all the units.
  
  @return string[]