A file with .atx extension is an ESRI ArcGIS file that stores ArcGIS 8 attribute indexes. There is a separate ATX file for each shapefile or dBase attribute index created in ArcCatalog. It is to be noted that ArcView GIS 3.x attribute indexes for shapefiles and dBase files are not used by ArcMap or ArcCatalog. However, these ArcView GIS 3.x attribute indexes can be converted to ArcGIS 8 attribute indexes using an ArcObjects tool.
ATX File Format - More Information
There are no file format specifications available for ATX file format to facilitate developers.