Choose “Not communicated” if B gave A no (EU) VAT number; transport is then attributed to A→B by default.
Explanation
Art. 36a applies only to VAT numbers issued by EU member states. Non-EU numbers (e.g. UK VAT number post-Brexit) count as ‘not communicated’. Exception (Art. 36a(2)): only a VAT number of the DEPARTURE country → transport attributed to B→C.