Source: Schema.org v30.1 vocabulary. Official addOn property definition.
What does addOn mean?
An additional offer that can only be obtained in combination with the first base offer (e.g. supplements and extensions that are available for a surcharge).
Used on types (domainIncludes)
Expected value types (rangeIncludes)
Schema.org domainIncludes and rangeIncludes are descriptive guidance, not strict validation prohibitions. Check the official reference for updates.
Related reference
Browse all Schema.org types and properties · Google feature support status