Who a step is about.
A step that acts on players says who: the player of the event, everyone, everyone else, a random player, the nearest. A step inside a loop over players is about that player.
Lists and selections.
Everyone is a list. Where narrows a list to those who pass a check, naming each one while the check runs; the nearest of, the farthest, a few at random, and sorting by distance work on any list, mobs and blocks included.
Teams.
A challenge declares teams with a name, a colour, and whether teammates can hurt each other. Steps put players on them and read who is on which. Everyone in the challenge is still the team wherever a step means everyone.