Learning the theory of Langlands correspondence, I met the notion of "special" maximal compact subgroup of a (reductive) algebraic group over a local field. Here, I think the word "compact" is used in analytic meaning. The textbook I used did not explain the definition of "special". My questions: (1) What is the definition of "special" maximal compact subgroup? (2) Is there any concrete example of maximal compact subgroup which is NOT "special"? Please give me any advice.