There are three ways of defining the used coordinate reference system:
EPSG -code
PROJ.4 -string
WKT version 1 -string
EPSG -code is a numberic value, which defines the coordinate reference system. If unsure which EPSG code is being used for the project, EPSG.io -web page is a great source to search for systems.
PROJ.4 and WKT allow custom localization for the coordinate systems, because it enables changes to the towgs84 -transformation values. PROJ.4 and WKT -strings are widely used by other similar systems. These can also be fetched from external web pages.
Area of use restricts the map and this allows the map interface to work faster.