That depends on the case, but most tax files begin with the core records: prior-year returns, ID details, income documents, account statements, and anything connected to foreign income, foreign taxes, business activity, or IRS notices. For expat and cross-border work, it is often helpful to include foreign payslips, tax assessments, bank or brokerage summaries, and any prior U.S. filing history. The clearer the records, the cleaner and faster the filing process tends to be.