What the challenge remembers.
A variable holds a value for the whole challenge or one per player: a number of lives, the player who is it, a place, a list of mobs. Steps set them, add to them, and clear them, and an event fires when one changes.
Per player, within bounds.
A per-player variable has a value for each player, and a step changes the player's own unless it says whose. A number keeps its minimum and maximum, and counts in whole numbers unless it is told to keep decimals.
On the sidebar.
A variable with a sidebar label shows there, under the goals, and each player sees their own value of a per-player one.
Goals from variables.
A goal can be a number variable reaching a figure: in total across the team, for any player, or for every player.