'External parties' are those who bear a cost or receive a benefit from an activity without choosing to participate in it — the standard economic definition of an externality. A clearly external party is identifiable and did not participate in the transaction at all (pollution downwind, systemic risk shifted to deposit-insurance fund, coerced taxation). A clearly internal party freely entered an arrangement that priced the cost in (an insurance pool, a club, a negotiated contract). Contested cases include future generations (treated as external via Hartwick's rule) and ecosystems (treated as external via shadow prices on natural capital).