A shelf offering can be used for sales of new securities by the issuing company (primary offerings), resales of outstanding securities (secondary offerings), or a combination of both (mixed offerings). Companies that issue new shares can register a shelf offering up to three years in advance, which effectively gives the issuing company that amount of time to sell the shares in the shelf. Depending on the type of security and the nature of the issuer, forms S-3 or F-3 (foreign issuer) must be filed to make the shelf offering.
A follow-on public offering is a type of financing that consists of the sale of common shares to public investors after initial public offering (IPO)
In the case of the dilutive offering, the company's board of directors agrees to increase the shares outstanding for the purpose of selling more equity in the company. This new inflow of cash might be used to pay off some debt or used for needed company expansion. When new shares are created and then sold by the company, the number of shares outstanding increases and this causes dilution of earnings on a per share basis. Usually the gain of cash inflow from the sale is strategic and is considered positive for the longer term goals of the company. Some owners of the stock however may not view the event as favorably over a more short term valuation horizon.
The non-dilutive type of follow-on offering is when privately held shares are offered for sale by company directors or other insiders (such as venture capitalists) who may be looking to diversify their holdings. Because no new shares are created, the offering is not dilutive to existing shareholders, but the proceeds from the sale do not benefit the company in any way. Usually however, the increase in available shares allows more institutions to take non-trivial positions in the company. This resale of privately held shares increases the float and not the outstanding shares.
The S-3 itself has no impact on price action, however, the offerings which can be derived from an S-3 in the form of a takedown may have impacts depending on a number of factors such as offering type, offering size, and timing of the offering that is used.
If the three-year window draws close to expiring and the company hasn't sold all of the shares in the shelf offering, it can file replacement registration statements to extend it.