add info on extent

[skorasaurus]

The extent refers to the area that is being represented.

Unless you have otherwise specified otherwise, your coordinates are in the '3857' aka, web mercator.

You can use the extent to select a smaller area that you want selected, by selecting the BBOX (short for bounding box, bbox is the "XMIN,YMIN,XMAX,YMAX") Note that the order of the bbox is not the same for all geographic/mapping programs.

This is very handy if you are working with a very large extract (and database) while working on tilemill and want to increase your overall speed. If you are working on the entire country of france, you can select Tilemill to only use a very small area (Say the town of Lyon) to work on.

(refer to bboxfinder.com as well)

• figure out if there's any use cases differences when someone would use this instead of the project's settings.