Here’s what is likely happening, which explains why it’s an "interesting feature":
Also used for the electronic residence permit fee, typically paid to account 67422402. bollettino td 674 pdf
There is no official Italian tax or postal form called "Bollettino TD 674" . It is almost certainly a misreading or software-generated code attached to a Certificazione Unica (ex CUD) PDF. Here’s what is likely happening, which explains why