VBX (Visual Basic Custom Control)
The VBX Developer file format
Files with a VBX extension are most commonly associated with Microsoft Visual Basic, Microsoft’s third-generation IDE (Integrated Development Environment). VBX files function as extensions of already existing Visual Basic scripts and programs to give them additional capabilities or expand their functions. They were introduced with the Control Development Kit (CDK) that comes with Visual Basic. Debugging VBX files can be done using either CodeView or Windbg (Windows debugger).
Technical details of VBX files
Different programs within the Visual Basic IDE can reference to VBX files. Among those are MS Visual Basic itself and WScript. Outside of Visual Basic, VBX files can be used by other programming systems by Microsoft such as Visual C++. VBX files can be considered deprecated by now. Instead of VBX controls, now mostly files with an OCX extension are used.