@Tourneur Hi, you are basically right about the first part. Only REXYGEN Studio is not a control program. It is a tool in which a program can be created.
To the second part: The index.hmi.svg visualization file contains the connection strings you set there. No additional index.hmi.js is needed (it's a different visualization type). It would be great if you went through the process of creating a simple example, which is described in the instructions, the link of which I sent in the last post. There is also a chapter on HMI: https://www.rexygen.com/doc/ENGLISH/MANUALS/RexygenGettingStarted_RasPi/RexygenGettingStarted_RasPi_ENG.html#x1-40000A
P.S: You will need to program for data communication via RS232. Communication will need to be set up in the REXYGEN or Python block.
Cheers,
Jan