Formation is a vital game mechanic of Inferno Legend.
Inferno Legend allows players to select Dark Servitors (e.g. Imp) to accompany their main character in battle. You initially start with only one slot for your main character, but as your Book of Sin level increases you will gain up to 7 more ending up with an 8-unit team. The arrangement in which they stand is a Formation. Each position in the formation has a number assigned. The target of attacks in battle is dependant on the skills used.
Looking at the formation setup screen (as seen in the posted screenshot), the lower left slot (No. 8 in the picture) is the main Front spot, and the upper right (No. 1) is the main Rear spot. As you move up the row to No. 5, that is the Front row which Front targeting spells will aim for first. If no more servitors exist in the Front row it will then move to the bottom spot of the rear row (No. 4). Area of Effect skills (AoE) that target Front will start with the front-most servitor and then move down the order until they have reached the maximum number of targets they hit. Rear-targeting skills start at the rear-most spot, No. 1. Center targeting skills start at the border spots between the two rows, either 4 or 5 with a full set of 8 living units, this has to be verified.
Normal attacks and the vast majority of the ultra-strong skills available target the Front, so this is where you'll want to put heavily defensive servitors (or your main character) to try and keep the rest of your team alive as long as possible.
Players can change Formation at any time while not in combat, including while enrolled in a Tournament, though while in a Tournament a formation switch won't occur until after your team fully dies within a round.
Sin level Edit
Players begin the game with access to only one slot, with a second quickly given during the tutorial, one of which must be filled by their main character. By leveling the Book of Sin, players can increase this limit to 8 slots, counting their main character.