The March CPS files include no data on asset holdings of any kind. The amount of financial assets can be imputed from asset income: interest, dividends, and rental income. Asset income is converted to asset holdings by assuming that all assets generate income and that all assets have the same rate of return. That rate of return is a program rule in each of the TRIM3 modules performing the assets imputation. Nonfinancial assets tests, such as limits on the value of an automobile, are not simulated in TRIM3.
http://trim3.urban.org/documentation/Steps%20in%20Simulating%20Tax%20and%20Transfer%20Programs.php