The Report() function only returns the reference to the Display Tree from the output from a Platform or a window created in a script. So by itself, it does not change anything. However, a piece of code such as
ss = Report( Eval( FCC_lims ) )[axisbox( 1 )];
ss << Min( slidelow );
creates a reference to an Axis in the Display Tree referenced by the 3rd element in the list FCC_lims, and then sets the minimum value of the Axis to whatever the value of slidelow is. And slidelow is set by the Slider Range Box referenced by SB2. Moving of the slider, will change the minimum value of the Axis.