Rule 811.4 · Alternating Teams Variant
A player can’t attack opponents who aren’t seated next to them, planeswalkers that aren’t controlled by opponents seated next to them, or battles that aren’t protected by opponents seated next to them.
Ask the judge Pro
The life counter has an AI judge that answers your play citing the exact rules. It is part of Gatewatch Pro; the rules, Oracle text and rulings stay free.