90% test for QOF purposes means that they have to have 90% of all assets in QOZBP (assets). There is a clear definition of QOZBP in the code and regulations. It's basically tangible property located physically within the geographic borders of a zone (i.e., with an actual office location where these assets are physically located and, hence, evidence would be the lease agreement for the office, property tax bills, deeds of trust for real estate) or equity investments in QOZ businesses that have at least 70% of their assets as QOZBP, so the same document ideas would apply to the QOZ business. You won't attach these items to the return, but they would be available for inspection in the event of audit.