One condition in a segment filter. field and operator are stable string keys from a server-owned catalog (kept as strings, not enums, so the vocabulary can grow without a schema change). Examples:

• field: "metadata.tier", operator: "eq", value: "gold"
• field: "metadata.tier", operator: "in", values: ["gold", "platinum"]
• field: "metadata.newsletter", operator: "exists"
• field: "signupDate", operator: "after", value: "2026-01-01"
• field: "registrationCount", operator: "gte", value: "5"

value carries single-operand operators; values carries set operators like in. Unary operators (exists) need neither.