FAQ: Which format can have ExternalData when importing CAD file?

An external is a subassembly tree that has been created in a different CAD system than its parent file. Externals are represented by a product occurrence node.

You can have A3DAsmProductOccurrenceData::m_pExternalData with Catia V5, step, stepxml and NX.
https://docs.techsoft3d.com/exchange/latest/guide/product_occurrences.html?#entities

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