Maps and scenes use coordinate systems to display data. A coordinate system is a reference framework that defines the position of features in either two- or three-dimensional space. Coordinate systems can be of different types, such as geographic and projected. A geographic coordinate system is based on a three-dimensional spherical surface and locations defined using degrees of longitude and latitude. A projected coordinate system is a planar system that uses two-dimensional coordinates and more often uses distance measurements as units. When a new map or scene is created, the default coordinate system is WGS 1984 Web Mercator. For a global scene, the coordinate system can’t be changed. You can choose a different coordinate system for maps and local scenes. As you add layers, they are automatically displayed using the same coordinate system as the map or scene. The map or scene’s coordinate system need not be the same as the data you are using, because it will project data on the fly. Be aware, however, that projecting on the fly can take longer to draw. to see details about the coordinate system. You can filter the coordinate systems shown in the dialog box by performing a search on a string value. This filters the list of coordinate systems based on folder names, coordinate system names, or well-known IDs (WKID). When using the filter options to reduce the number of available coordinate systems, only the folders containing the matching coordinate systems will appear. Either browse to the coordinate system you require in the list, or type part of the name of the coordinate system you are trying to locate in the folder and expand the coordinate system. When the coordinate system is expanded, you’ll see the layers that reference it. This is a useful way to ensure all layers in your map are in the same coordinate system. Source.