I wanted to highlight the difficulty in discerning the rules for the community here.

A lot of the content here is motivated via 'homework' type questions. People have trouble with the maths behind work they're assigned, and with implementing said maths in Mathematica. In one extreme, people identify the core mechanism they are struggling to implement, and seek Mathematica specific advice. Presuming they've failed to Google solutions using their terminology, each post ties new descriptions of problems to solutions, expanding what future Googlers can achieve in Mathematica. The other extreme is perhaps more frequent. People join the community to ask a singular question, and the question is left firmly in the setting they found it in. "I've copied twenty equations out of a text book expecting divine intervention, what went wrong in these 50 lines of code?". These posts require more work to answer, and are less helpful to future readers.

I've heard (if not witnessed) that homework is frowned upon, but one has to dig to discern this. There's some discussion here: What if a post is homework?

What are the rules? Why aren't there definite guidelines for users displayed prominently?

One has to dig into the community guidelines to get a gist for the intended content; they're extremely unspecific, and not naturally come across. Simple, prominently featured, instructions may indicate to posters that problems are best when specified out of context, in small examples. This in turn might prompt people to try and isolate their problems in short code, which is often their solution anyway!

A tangential comment on the interface: The 'flag' option on posts references 'liferay.com', who provide the software behind this community, where it likely shouldn't.