The purpose of Form 990-T is to report an organization's unrelated business income. If your organization has received a gross income of $1000 or more from an unknown business for the tax year, then you must file Form 990-T along with your 990 series return.
For example, the investment income from debt-financed property unrelated to the organization's charitable purpose and certain rents (and related expenses) is treated as an unrelated trade or business income and should be reported by the organization on Form 990-T.