In steel design we distinguish between yield strength $f_y$ and ultimate strength $f_u$. We use also ultimate strength of a member which means that a member is plasticized at some part.
If a member is designed with plastic analysis and lateral-torsional buckling might be involved, then such buckling has to be prevented by restraints located at plastic hinges. Member has to be of class 1.
If it is the bottom flange which is being under compression then the restraint is needed for the bottom flange (and vice versa).
To prevent local buckling in bending the constraint has to prevent compression flange from moving to the sides.
The diagonal brace preventing lateral displacement is designed to resist 2.5 %of $N_{f,Ed}$. $N_{f,Ed}$ is axial force in compressed flange of the stabilized member at the plastic hinge location. That means that 2.5 % of the axial force is able to prevent local buckling to the side.
Length between segments has to be not greater than the stable length. For I, H sections with $h/t_f\leq 40 \epsilon,\ \epsilon = \sqrt{235/f_y}$:
$$ \begin{align} L_{stable} &= 35 \epsilon i_z \hskip2em &0.625 &\leq \psi \leq1 ,& \\ L_{stable} &= (60-40\psi) \epsilon i_z \hskip2em &-1 &\leq \psi \leq 0.625,& \\ \psi &= \frac{M_{Ed,\min}}{M_{pl,Rd}}, \end{align} $$and $M_{Ed,\min}$ is minimum moment within the member, $M_{pl,Rd}$ is ultimate (plastic) resistance.