Function register_external 

pub fn register_external(
    conn: &Connection,
    pepper: &[u8; 32],
    values: &[String],
    mode: Normalise,
    meta: &BatchMeta<'_>,
) -> Result<Vec<RegisterOutcome>, String>

Registers an ID that was minted elsewhere — the provenance-recording half, distinct from mint which generates in-process.