what is Types of Offerings differant from common type?

Specific Types of Offerings 

In a registered secondary offering the key word is secondary. As with all secondary or non-issuer transactions, the proceeds are not going to the issuer. Rather, they are going to, for example, a former CEO or board member who is now offering his or her restricted shares to the public. The restricted shares were not registered; now they are being registered and offered to investors on the secondary market. Remember that if the issuing corporation does an additional offer of stock, it is not a secondary offering. Rather, it is a "subsequent primary distribution." When the issuer gets the proceeds, the word is "primary," not "secondary."

A specific type of firm commitment is called a standby underwriting. While a broker-dealer cannot buy IPO shares for its own account just because it wants to, they can act as a standby purchaser for the issuer, buying any shares the public doesn't.

Usually, we associate "standby" with an additional offering of stock, which inherently involves a rights offering.

Shareholders are owners of a certain percent of a company's profits; therefore, if new shares are sold to other people, the % owned by existing shareholders would be decreased or "diluted" if they didn't get first right of refusal on a certain number of shares. That's why issuers performing an additional offering of stock typically do a rights offering that provides existing shareholders the right to buy their % of the new shares.

To ensure that all rights are used/subscribed to, a standby underwriter may be engaged to agree to buy any rights that shareholders don't use and exercise them to buy the rest of the shares being offered.

Some offerings of securities are registered now but will be sold gradually at the current market price. That means if you buy in the first round, you might end up paying more, or less, than the investors who buy shares at the then-current "market price."

What if there is no "market" for the shares being offered in an at the market offering? 
Then, the SEC has a real problem with broker-dealers or their associated persons telling investors they're buying the security "at the market."

If the stock doesn't trade on an exchange, the broker-dealer may not tell the customer the security is being offered to (or purchased from) the customer at the so-called "market price." That would be a "manipulative, deceptive, or other fraudulent device or contrivance," according to the SEC. If the firm is the only firm willing to make a bid / offer on the stock, that, by definition, means there is no actual market for the security.

An issuer might want to register a certain number of securities now but sell them gradually or on demand over the next few years. If so, the issuer can use a shelf registration. For example, if they want to borrow money by issuing bonds, they might want to get them registered now but wait and hope that interest rates will drop over the next few years, at which point they can issue the bonds and borrow the money at more attractive rates in the future, with the offering already on-deck and ready to go. Or, if the company has a dividend reinvestment program (DRIP) in place, or must continuously issue shares when executives and key employees exercise stock options, they are likely to use a shelf registration.

The term "when-issued" is an abbreviation for the longer form of securities that are traded "when, as, and if issued." As the name implies, when-issued refers to a Transaction made conditionally, because a security has not yet been issued only. U.S. Treasury securities sold at auctions new issues of stocks and bonds, and stocks that are offered continuously or over time are all examples of when-issued securities.