what is actually "EXTENT" in gis slang?

I am not clearly understanding the extent parameter in GIS applications. for ex, In mapserver map file we are using

NAME "CGI-CONTEXT-DEMO"
STATUS ON
SIZE 400 300
**EXTENT -2200000 -712631 3072800 38400开发者_如何学C00**
UNITS METERS
IMAGECOLOR 255 255 255
IMAGETYPE png

here extent means lower left x,y and upper right x,y. but long,lat values are not used here , then what is this value? How it is arrived?

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Extent is simply an area(square) which is specified with left lower and right upper x and y. In your example it doesn't look like lon lat because it's just a different coordinate system.

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The minimum bounding rectangle (xmin, ymin and xmax, ymax) defined by coordinate pairs of a data source. All coordinates for the data source fall within this boundary.

Source: http://support.esri.com/en/knowledgebase/GISDictionary/term/extent (GIS dictionary)