For a SPO (Secondary Public Offering), which involves the sale of additional shares by existing shareholders or the company itself after its IPO (Initial Public Offering), the specific form the issuer files with the U.S. Securities and Exchange Commission (SEC) depends on various factors, including the nature of the offering and the status of the selling shareholders. Here are some common forms used for secondary offerings:  

• Form S-1: While Form S-1 is typically associated with IPOs, it can also be used for secondary offerings if the selling shareholders are not eligible to use Form S-3. Form S-1 requires comprehensive disclosures about the issuer, the selling shareholders, and the offering itself.  

• Form S-3: If the selling shareholders meet certain eligibility criteria, they can use Form S-3 for secondary offerings. This form is a streamlined registration statement that allows eligible issuers to incorporate by reference their existing SEC filings, simplifying the registration process.  
• Form S-4: In the case of mergers, acquisitions, or exchange offers, Form S-4 may be used for secondary offerings of securities. This form is used to register securities to be issued in connection with these transactions and requires detailed disclosure of the terms and conditions of the offer.  
• Form F-3: Foreign private issuers may use Form F-3 for secondary offerings of securities. It is somewhat like Form S-3 but tailored for non-U.S. companies that have listed securities in the U.S.