Filing History Items - Unique Identifier

Is it safe to use the Transaction ID as a unique identifier for a filing history item?

I had been assuming that was the case, however in the resource representation it is marked as an “optional” field. This suggests that not all filing history items have a transaction ID and it can’t be used. Is this the case or is it an error in the documentation? If not all filing history items have a transaction ID, is there an alternative value I should be using?

Ash,

The transaction_id is a unique identifier and the documentation is in error.

Thanks

Mark.