Looks simple and trivial and that is the power of documentation (that I can't stress more). In star positions phase, as mentioned positions for the stars are calculated but stars are not yet generated. In the next phase
star types and sizes are decided. There potentially could be more attributes such as space monster lairs or alien relics but at the moment they are not defined. In parallel to that phase can go the phase where "starlanes" (or wormholes) are distributed. Planet distribution phase and planet attributes phase may actually be single phase. Anyway those two phases fill star systems with planets and calculate attributes such as planet size or environmental conditions.
Potentially each phase on the flow chart can be made moddable but at the moment I don't see the need for that extreme. For now I'd group those five phases to two groups: star graph and planet generation.
Odd choice is putting star attribution to planet generation. The original idea was that star type directly influences distribution of planets (both positionwise and typewise) in the star system. Putting star positions and starlanes in the same bucket may reduce the code reuse a bit but it simplifies interfaces for plugin code. I don't know, may I could split them somehow without "void *" arguments and with the fewest presumptions.