"Code" is generally more in line with "rules" (plural), as it is referencing a system or set of rules/regulations/guiding principals. It could be used singularly, but that's less correct, even if it is acceptable in pretty much any casual scenario.
The teacher has a lot of rules.
The teacher has a strict code of conduct in his classroom.
Some people prefer to pay a professional to file their taxes for them because of the complicated tax code.
The written and unwritten rules of the game make up a long-standing code that many baseball players take seriously.
The Language Level symbol shows a user's proficiency in the languages they're interested in. Setting your Language Level helps other users provide you with answers that aren't too complex or too simple.
Has difficulty understanding even short answers in this language.
Can ask simple questions and can understand simple answers.
Can ask all types of general questions and can understand longer answers.
Can understand long, complex answers.
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