Changing the value in the Seed field will generate the corresponding input to the Input field.
Pressing the Download button will download the inputs for seed values seed, seed+1, ..., seed+#cases-1 at once.

By running your program locally against the generated input and pasting the output of your program into the Output field, the visualization results will be displayed.
Pressing the ▶ button starts animation.
By pressing the Save as PNG button, you can download the visualization results.

You can quickly switch between multiple outputs by pressing the Select File button at the top and selecting the directory containing the output files.
By naming the output file 1234.txt or abcd_1234.txt, the seed number (in this case 1234) is automatically set when the file is selected.

Hovering over each rectangle will display information about it (number, occupied coordination, true size, estimated size).
When "Hide number" is checked, the number of each rectangle will not be displayed.
When "Show scale" is checked, scales with an interval of 100,000 will be displayed at the left and top of whole arrangement.
When "Show estimated size (box)" is checked, the estimated size of each rectangle will be overlaied (top-left aligned).
When "Show estimated size (frame)" is checked, the estimated size of whole arrangement will be displayed.