If your Solo 401k trust purchases an investment such as real estate, you may be required to complete form W9.
Form W-9 is used by both individuals and entities to provide their taxpayer identification number (TIN or EIN) to entities who will pay them during the tax year.
Entities will use form W-9 for accounting purposes, but generally don't send a copy of the form to the IRS. The information on form W9 is then used to prepare IRS form 1099-MISC at the end of the year to document payments.
If your Solo 401k trust completes this form, and receives form 1099-MISC, it doesn't mean you have to pay any taxes, but you may still need to complete a tax return for your entity. The tax return would be informational only as your original EIN (employer identification number) for your Solo 401k Trust is connected to and owned by a tax-deferred or tax-exempt entity (your retirement account).