Direct Form 2 IIR filter input values In the filter graph below:




where circle with 'X' inside is a multiplication
circle with 'Σ' inside addition
and the triangle is a delay element


What could be the values x(k) in this type of filter? There is a rule? There is a range? I can understand the matrix of coefficients but i don't know about the x(k) values.
$x[k]$ is the input signal to the filter; it could be anything. There is no "rule" for what signal you can input to a linear filter. With that said, the stability of filters is usually talked about in a bounded-in, bounded-out (BIBO) sense. If the input signal is unbounded, then it's likely that the output signal will be also. So if anything, the "rule" that you're looking for would be to only apply signals $x[k]$ whose magnitude $|x[k]|$ is bounded below some value $B &lt; \infty$.