ATS stands for Alternative Trading System.
Regulatory change in recent years has brought the ATS into existence.
An ATS is an after-market exchange where people that own securities can buy and sell.
Because all Reg A+ offerings are Public Offerings, listing Reg A+ securities after the Reg A+ completes on an ATS is a useful way for a company to provide liquidy to its investors.
One big advantage of ATS exchanges is that shorting of stocks is not possible and naked shorting is not possible.
The fees to list are approximately $10k plus the cost of filing with the States for secondary market Blue Sky exemption.