Hi @slavo,
client_reference is capped at 191 characters.
You should not be constrained by the comment field limit but keep in mind that it has to be readable for the driver.
The client_reference is unique to avoid duplicate orders and ensure traceability. But after a delivery is canceled you can used the same client_reference again as the unicity is only for active deliveries. More details on Why should I specify a `client_reference` in the Job creation?