what the codec enforces — its limit, its values, or neither.
the field's travel belongs to the INSTANCE (a preamp's gain span, an appliance's own enum gate), so no number may be published here. The codec's fallback is deliberately NOT published: a plausible wrong travel is the defect this withholding exists to prevent (FieldTravel.limit). A VOCABULARY still is — its members are fixed when the codec is built.
One field's row-wide travel, or
undefinedwhere the field declares none.undefinedis a THIRD answer, distinct from{}: a bool, a free token and a derived field all have nothing to declare, and a client reads their absence as "this field has no numeric or enumerated travel" rather than as "ask the instance".