> For the complete documentation index, see [llms.txt](https://infosys.selmotech.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://infosys.selmotech.com/selmo-solution/selmo-solution-en/selmo-how-it-works/selmo-training/chapter-4-the-ptf-principle-process-technology-function/4.1-why-ptf.md).
# 4.1 Why PTF?
Selmo is not a tool, but a method.\
The crucial difference from classical PLC programming is that the sequence of a machine is not first conceived in the code, but already in the analysis – in three successive steps: **process**, **technology**, **Function.**
PTF is thus the bridge between reality and model.\
It ensures that the logical structure of a machine is always derived from the real behavior – not the other way around.
>
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