For QuickBooks Online: Sumac tries to send every record to QuickBooks Online one at a time. Ones that succeed get assigned transaction IDs, ones that fail do not. There is a message at the end that tells you how many succeeded and how many failed.
For QuickBooks Desktop: All transactions are sent at once in one big batch. QuickBooks Desktop will return transaction IDs for the ones that worked, not for the ones that didn't. If some failed, there is an error message.
With both the online and desktop versions, it is always possible that Sumac will get a terminal error that prevents it from sending anything further (e.g. the connection between Sumac and QuickBooks may be dropped). If this occurs there is an error message that tells you what happened and, as above, the transaction IDs will show which ones succeeded and which ones failed.