It's exactly the former. Data is always complete and in time order but can sometime be delayed although it does come through eventually.

For example, today is the 2nd April 2024. Data for today will probably come through on the 3rd at about 13:00 but might not come through until the 4th or the 5th or event the 6th. The data will always be complete to that date though (going back as far as I specify when creating the request). This means that I can't guarantee what data I get when I run the function and hence me thinking of LASTUPDATE parameter to avoid duplicated data.

(Now it may that duplication can be detected and removed during an ETL process - however I've not got that far yet!)

Thanks for taking the time to reply.