You can edit default .sdx_def files or specify a pre-defined schema with specialized LandXML and User-defined properties contained in an existing *.sdx_def file.
By default, AutoCAD Civil 3D Survey installs the LandXML - Standard .sdx_def and Sample .sdx_def files at the following location:
C:\ProgramData\Autodesk\C3D <version>\enu\Survey
For example, the LandXML - Standard .sdx_def file contains numerous SurveyFeatureClass items. You can open this file in a text editor and make any necessary edits to streamline this file to meet your needs. Use the Save As command and then save the file with a new name.