The <box> element has the following definition:

<!ELEMENT box (coordinates) >
<!ATTLIST box
  ID CDATA #IMPLIED
  srsName CDATA #REQUIRED
  preserve_aspect_ratio (TRUE|FALSE) "FALSE"
>

The <box> element is used to specify the bounding box of a resulting map. It uses a <coordinates> element to specify two coordinate value pairs that identify the lower-left and upper-right corners of the rectangle. The coordinate values are interpreted in terms of the user's data. For example, if the user's data is geodetic and is specified in decimal degrees of longitude and latitude, a <coordinates> specification of -72.84, 41.67, -70.88, 42.70 indicates a bounding box with the lower-left corner at longitude-latitude coordinates (-72.84, 41.67) and the upper-right corner at coordinates (-70.88, 42.70), which are in the New England region of the United States. However, if the data is projected with meter as its unit of measurement, the coordinate values are interpreted in meters.

preserve_aspect_ratio is an optional attribute that indicates whether or not the box coordinates should be further modified so that it has the same aspect ratio as the map image or device. The default is FALSE, in order to keep compatibility with previous versions that do not have this attribute. If this value is set to TRUE, the box is modified to preserve the aspect ratio, so as not to distort the resulting map image.