Source Coordinate System: This is the source coordinate system of the well files you are importing. Select a coordinate system by clicking . By default this is set to the coordinate system in the project properties dialog. If you have obtained input files that are based on a different coordinate system from that of the project, set this to automatically perform coordinate conversion. If you do not have a coordinate system defined for the project, this will be blank.

Destination Coordinate System: This is the destination coordinate system for the project. Select a coordinate system by clicking . By default this is set to the coordinate system in the project properties dialog. It is best to set this in the project properties rather than in this dialog.
If you do not have a coordinate system defined for the project, this will be blank.

Assume western longitudesTurn this on to specify that all longitudes are in the western hemisphere.  Many older Photon ASCII files will have positive longitudes for data in the western hemisphere.  Turning this on ensures that values are correctly converted to map units. Turn this off if your longitudes are actually in the eastern hemisphere.