It is the gross receipt of the organization that determines the kind of Form 990 they must file unless it is a Private Foundation.
Private Foundations, regardless of their financial status, should file the annual tax return Form 990-PF.
Any exempt organization that has earned a gross income of $1,000 or more from an unrelated business must file Form 990-T along with a 990 series form.
All these forms mentioned above are due by the 15th day of the 5th month after the tax period ends. If in case, an organization misses this deadline, they can request for an automatic extension of 6-months by filing Form 8868. This extension of time can be sought for all forms in the 990 series, except Form 990-N.