This information is part of the Modelica Standard Library maintained by the Modelica Association.

The block has a vector of Boolean input signals u[nu] and a vector of (time varying) Boolean expressions expr[:]. The output signal y is set to expr[i], if i is the first element in the input vector u that is true. If all input signals are false, y is set to parameter "y_default" or the previous value of y is kept if parameter use_pre_as_default = true:

// Conceptual equation (not valid Modelica)
i = 'first element of u[:] that is true';
y = if i==0 then (if use_pre_as_default then pre(y)
                                        else y_default)
    else expr[i];

The usage is demonstrated, e.g., in example Modelica.Blocks.Examples.BooleanNetwork1.