Identity and civil documents for everyone who might be included, a complete record of prior U.S. immigration history, and evidence of where the funds are and whether they are genuinely available.
Identity, history, and real liquidity
Passports, birth certificates and marriage records come first, for each person who might be part of the application. Next comes history: every previous U.S. application, admission, refusal, withdrawal or period of overstay, with dates and outcomes, because admissibility is assessed on that record and an unresolved matter is much better identified early than discovered late. Then the funds. Shares acquired through employee options need the option agreements, exercise records, brokerage statements and tax filings; a registered plan needs its statements and its withdrawal rules. The question is not only where the money came from but whether it can be moved now. Finally, keep the dated copy of the official material you relied on, so that any decision can later be traced to what was actually published when it was taken.
The official process describes further USCIS instructions following the initial application, so the applicant should expect to produce the ordinary immigrant-petition evidence for whichever classification is assigned, plus whatever the program itself requests. The first review should identify what a document proves: a passport establishes identity and nationality; a payment receipt may establish that a required amount was transmitted; achievement records may support an EB-1 or EB-2 theory.