I tested out IRS's new Form 1042-S Data Integrity Tool at https://1042sdi.for.irs.gov/Account/LogIn?ReturnUrl=%2F
This software program provides a no-cost standalone tool to assist in complying with withholding reporting IRS Form 1042-S Foreign Person’s U.S. Source Income Subject to Withholding. The tool performs a quality review of filer's 1042-S as a pre-submission accuracy check enabling filer to make required corrections versus IRS upon its review, then afterward one files 1042-S with IRS in the regular manner and not thru the software checker where the form was uploaded.
One must complete and file the form using the tricky/extensive codes and definitions provided at https://www.irs.gov/pub/irs-pdf/i1042s.pdf
See this test result page for USCO that paid parent CANCO USD$400,000 dividend.