Use the extent geoprocessing control to set a geographic area of interest to be applied to a geoprocessing tool. This extent is defined as a rectangle with minimum and maximum X (longitude) and Y (latitude) coordinates. You can think about the coordinates that make this rectangle as left (minimum longitude), right (maximum longitude), top (maximum latitude), and bottom (minimum latitude). You can enter an extent equal to the left, bottom, right, and top coordinates of the rectangle, or get the extent from the active map, any map layers, or a geodataset on disk.
Sometimes the extent will define the area within which to create new data. For example, when you specify a constraining extent for the Create Random Points tool, all points will be inside the extent rectangle. When an extent is used to process feature or raster data, the extent rectangle is used to select, but not clip or cut, the data for processing (any feature or raster cell that is inside the extent rectangle will be processed in full).
Set the extent
You can enter coordinates into the extent coordinate boxes.
You can use a menu to set the extent equal to the current display extent or the extent of any layer in the active map. When you pick either of these options, the extent is read from the map or layer and put into the extent coordinate boxes.
If no map is open always be sure to specify the extent coordinates in the coordinate system of the input data. When a map is open, the extent should always be in the map's coordinate system, not the input data's.