Open 123DX File
The 123DX extension is a 3D model created by 123D Design, a program used to design models for printing or manufacturing.
About 123D Design
It was a hobbyist CAD and 3D modeling toolset created by Autodesk.
It is functionally similar to Trimble SketchUp and is based on Autodesk Inventor.
In addition to the most basic drawing and modeling capabilities, it also has assembly and constraint support and STL export.
The 123D Design software is no longer available for download as the developer stopped its support. Instead, there are a number of utilities and applications developed by AutoDesk, which replace all the functions of 123D Design.
Function of the 123DX file
The 123DX file stores the 3D geometry of the model, as well as the materials that define the appearance of the model's surface.
123D Design models, saved to a 123DX file, automatically assign connections between parts that give the model natural movements when moved.