How the User Report is written in relation to the User Report XML. It depends on if you're autoloading or writing to a dataset.
If you're writing XML to a file: ACR builds the XML in its work/buffer space as the report is being written to the normal report DD. Once the buffer reaches 2000 it writes one line of XML to the XML dataset and starts over (the LRECL of the UNIWXML is 2000).
If you're autoloading XML to the Insight database: ACR builds the XML in its work/buffer space as the report is being written to the normal report DD. It checks the user options file to ensure the number of bytes hasn't been exceeded.
For example, if you had the limit set to 10,000 bytes it would continue building the work/buffer space until that number was reached but then not autoload and instead write to the recovery file (optional).