A follow-on is executed between one close and the next open, and made possible months before. A listed issuer with twelve months of timely filings and a $75 million public float keeps a Form S-3 shelf on file, and a takedown needs no new SEC review: the deal launches after the close, prices overnight, and the supplement is filed under Rule 424(b) within two business days. A secondary adds a wrinkle: a well-known seasoned issuer may name selling shareholders in the supplement; anyone else must have identified the holders, or the transaction that gave them their shares, when the shelf was filed.
Speed changes nothing about liability. The supplement is part of the registration statement for Section 11 purposes, so the underwriters bring their diligence down in the same window, and the auditors' comfort letter gives negative assurance only on a period ended within 135 days, which decides whether a deal can price before the next quarter's numbers are out.
We put the shelf up in calm weather, keep registration-rights mechanics and board authorizations current, and act for the issuer or the selling holders, never both in one deal.