The Fusion Simulator calculates for all possible fusion combinations that can be created from the hand. The light-blue box is for fusing a card that is currently in your Summoning Area. It will place the card as being first in the fusion order. The order of cards for all other boxes does not matter.
Fusions involving "Insect Imitation" will be displayed as "?" and won't be taken account for other fusions due to the random nature of the card.
Enter the name or ID number of each card, but a minimum of 2 cards is required. Use this guide for a general fusion list.