In Fractional reserve banking, which is the common practice these days in most (?) states in the world, a bank is required to limit its loans to a certain multiple of its reserves.
I know that central banks typically set the multiple (i.e. the required reserve fraction). But - who decides on what constitutes reserve? And how reserve is counted? Supposedly, in the olden days of the Amsterdam goldsmiths, the notes were, say, for X amount of gold (or some other precious material) so the reserves were gold, and it all went by weight. But now we have physical currency, and balances with the central bank, and maybe precious materials, and maybe a whole bunch of other assets which a bank may hold. Who decides how they're counted/combined in the evaluation of the reserve?
This question is mostly about the larger economies, but actually I'm more interested in examples of how this gets decided (or if there's some universally-accepted rule, then in that).