What do I do with Form 3921 once Cake generates it?
Three copies, three destinations. Cake generates two of them and gives you the data for the third.
Copy B goes to the employee
Send it to every person who exercised ISOs in the tax year, including anyone who has since left. Find it under Documents → All documents → IRS → [Year] → Form 3921 - Copy B. They need it to work out Alternative Minimum Tax and set their cost basis.
Due 31 January, for the prior calendar year.
Copy C stays with you
Your company record of each exercise. Cake files it alongside Copy B, in the Form 3921 - Copy C folder. Nothing to send.
Copy A goes to the IRS
This is the one Cake does not file for you. Download the report from Compliance → Form 3921 and pass it to your accountant or tax software to prepare and submit Copy A.
Due 28 February on paper, 31 March if filing electronically. Filing 10 or more information returns in a year, counting W-2s and 1099s together with your 3921s, makes electronic filing mandatory.
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Form 3921, explained - filing methods in full