Where can I find documentation on the Visum COM API?
1) You can find an introduction to scripting with the Visum COM API in Help -> Introduction to the Visum COM-API^ leading to the file c:\Program Files\PTV Vision\PTV Visum 17\Doc\Eng\Introduction to the PTV Visum COM-API.pdf
2) All properties and methods are documented in: Help -> COM help and leads to the chapters 'Visum COM' and 'Visum - Python'.
3) The folder %Public%\Documents\PTV Vision\PTV Visum 17\COM\ includes sample scripts.
4) In the path %ProgramFiles%\PTV Vision\PTV Visum 17\Exe\AddIns you can find the source code of all AddIns being part of the standard installation. They are often a bit more advanced, but might be of interest when needing a template for an AddIn or if you want to adapt an existing AddIn.