I understand what you mean and it may be slightly misleading, but these buttons select the Nth scale note, so they do not indicate intervals (only in the case of the Chromatic scale). For major or minor scales, buttons VIII to XII are not used at all.
Traditionally roman numerals mean scale degrees in RapidComposer.
Please note that recently a new setting was added to select between alternative and conventional notation. I think you'd prefer the conventional notation:
![Image](https://snipboard.io/9FebgA.jpg)
Thanks,
Attila