A superseding return is a subsequent return filed within the filing period (including extensions). It is considered the return of record because it takes the place of any other return previously filed during the filing period, with extensions. An amended corporate return, on the other hand, is a subsequent return filed after the expiration of the filing period (including valid extensions).
Sources:
Publication 538 (01/2022)
Instructions for Form 1120 (2023)
Publication 4164 (10/2023)
Publication 4163 (12/2023)
IRM Part 3. Submission Processing. Chapter 11. Returns and Documents Analysis. Section 16. Corporate Income Tax Returns
