The federal ethics disclosures Mr. Trump has made about his holdings so far have been limited. For example, there is no requirement that he report the identities of investors who have put money into his businesses.

Mr. Trump acquired many properties, in particular golf courses, with cash, but where did that cash come from? Mr. Trump’s son Eric once told a journalist that the Trump Organization’s golf properties were paid for with Russian money. Eric Trump has denied making that statement.

While Mr. Trump’s federal filings have reported some outstanding loans, investments and income from each business, they do not include debts where he has a partial interest or older loans that have been paid off. In theory, the existence of those older loans — and the lenders behind them — could be disclosed on his historical tax returns because certain types of interest on debts can be tax-deductible.

In reality, much of that information is unlikely to be contained in the limited number of returns requested by Congress. The panel is seeking the returns for only a small number of Mr. Trump’s companies. But he controls hundreds of separate L.L.C.s that contain individual businesses, as varied as a particular building or the rights to license his name overseas. Each one of those businesses may file its own tax return, separate from the ones requested by the committee.

How exactly does Mr. Trump make his money?

Mr. Trump owns and operates businesses in the United States and overseas. He earns revenue from golf club memberships, from leasing space in his buildings and from licensing deals to put the “Trump” name on other developers’ properties.

Mr. Trump’s government disclosure reports show ranges of how much revenue his various subsidiaries collect, as provided by Mr. Trump, but they do not indicate the companies’ overall profitability.

For example, his most recent government ethics report states that his Irish golf business reported revenue of $14 million in 2017. However, Irish regulators require that the golf subsidiary make its filings public. Those disclosures show the business actually lost about $2 million that year. Mr. Trump’s returns could provide further detail about the profitability of his other businesses. More broadly, his tax returns can be compared with the ethics disclosures to check the accuracy of the filings.