Eligibility has two halves in the form's General Instruction I. The registrant requirements include having been an Exchange Act reporting company for at least twelve calendar months and having filed all required reports on time during that period. Late filings of certain 8-K items are excused, and a report filed within a Rule 12b-25 extension still counts as timely. It also must not have skipped preferred dividends or materially defaulted on debt or lease payments since its last audited fiscal year.
The transaction requirements determine what the company can sell. For a primary offering of any size, the market value of common equity held by non-affiliates must be $75 million or more. A company below that level that has a class of common equity listed on a national securities exchange and is not a shell company can still use the form, but its sales in any 12-month period are capped at one-third of its non-affiliate float. Well-known seasoned issuers get an automatic shelf that becomes effective on filing.
After an S-3 shelf is effective, each sale is documented in a prospectus supplement filed under Rule 424(b). For investors, a newly effective S-3 is a signal of capacity, not a sale: it tells you the company has pre-cleared the ability to issue stock or debt quickly. The supplements show when that capacity is actually used, and repeated takedowns are a common source of share-count dilution.