What is the “individual number” as mentioned in the EU VAT Directive?

Article 214(1)(d) of the VAT Directive requires Member States to identify taxable persons liable to pay VAT with an individual number.
There is confusion regarding whether this number is the VAT identification number, causing uncertainty for suppliers making taxable supplies to persons in other Member States.
Some Member States do not register exempt persons liable to pay tax under reverse charge, leaving suppliers at risk.
The European Commission needs to address this issue for legal certainty.