Let's go back to basics, shall we? The constitutional purpose of the Census is to enumerate the population for the purpose of determining representation. Period. There is no other purpose. All "persons" are counted, with the specific exclusion of non-resident aliens (which includes visa holders and diplomatic personnel). That's it.
Congress has also authorized, by law, asking demographic and other information for statistical purposes. However, these additional queries may not interfere with the constitutional purpose, enumeration. Constitution supersedes law (it's in the Constitution).
The "short form" goes to everyone, and is really just about enumeration (although it asks additional questions, it really is short). The "long form" goes to a representative sample of households to get additional[/I ]informantion Congress had deemed "useful". The short form has NOT included a citizenship question since 1950, because it is not relevant for enumeration.
That it's the context for the question before the court. There are three issues in the court case: 1) How was the citizenship question added (process), and did it violate the Administrative Procedures Act? 2) Why was it added - was it for an impermissible purpose, and/or as a pretext for an impermissible purpose? And, now, 3) does it interfere with the Constitutional purpose of enumeration (by making the count inaccurate)?