From the manual, it depends on "Machine Type" In the configuration menu:
Machine: Allows be specification of the general type of CNC machine being controlled. This setting affects how TurboCNC interprets RS-274 D Code as follows:
• Radius Lathe: The X axis (cross feed) is used as specified when processing a move. This default plane is set to G18 at startup.
• Diameter lathe: The X axis (cross feed) is halved internally before processing a move. Earlier versions of TurboCNC had a cryptic way of setting this via an AxisPreScale parameter in the ini file. This default plane is set to G18 at startup.
Under the General config. menu, there is a drop down menu and you select the appropriate type. Mine is showing Mill/Drill right now, but you can change that. Don't forget to save the config changes and then reset the ports!