I see your confusion. I think they are asking for two circuits - 240v for the optional heater, and 120v for lights and such. However, I am not sure. I did not see where one could download or view the actual installation instructions. If not available online, suggest you contact the vendor directly and ask for clarifiction.
The dual amperage requirements is a bit confusing. Usually it refers to either 120 or 240 volt sources, but it really doesn't indicate that in the drawing or the specs. I'd expect one to be 120 and the other 240. Do what a "professional" (or master) would do and call the mfg. and ask. It doesn't ever hurt to ask the questions, regardless of your expertise. I've seen experts burn up so many things because they "thought" they knew.
The specifications seem to say that it requires 11.8A at 120V, and also 13.6A at 240V. So you need two dedicated circuits for this thing. The NEC would allow the 120V circuit to be a 15A, and the 240V to be 20A. I can't tell for sure from the diagram, but chances are you'll need to install these receptacles inside the wall (mounted sideways on a stud), and have an access panel on the other side, since this shower does not appear to be a removable appliance.
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