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.
So that's why I'm careful with the terms. System is what you're really doing composed of dozens of Techniques. Rules are what's in the book, or whatever other text is involved. They ain't the same things.
And even more important
Let's go ahead and say there's a rule in the book, and that it is indeed what we follow at the table. A bastard sword used two-handed does 3d6 damage - well and good. But why? How is it that we do that?
We don't do it because "it's a rule." Rules do not and cannot make people do things. We do it because we want to do it that way. They are used by people to do things, either well or badly.
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.