Right ... everyone knows what the rules are, right?
Except that (1) no one present actually read the book or knows any of the procedures in it from the text, except for one or two people. And (2) what the group actually does at the table may not resemble that text at all, no matter how much the people claim otherwise. Finally, (3) in some cases, what some people think is being done isn't actually what's being done.
Textual instruction about (a) anything and everything concerning role-playing this particular game, or (b) specifically Techniques and Ephemera. Used in this sense, Rules are distinct from the System actually employed during play, although it may be used as a reference or justification for it.