Tag

The Tag Feature allows you to associate a maximum 64 bytes long indexation tag with an output. Similarly to the Metadata Feature, the actual Tag is treated as binary data by the protocol.

Note that, by default, nodes do not store and expose the indexation by tag. However, the Indexer API does. In an "IOTA as a payment method" use case the Tag field could be used as a payment reference field, much like the payment reference option of a regular wire transfer.