This is a new exemption released by the SEC on September 23rd, 2013. Like Rule 506(c), startups using this exemption can raise an unlimited amount of capital from Accredited Investors . But unlike 506(b) these startups can advertise their fundraising to the public.
There's a downside. Startups using 506(c) must verify that all their investors actually are accredited. This may require the investor to provide a letter from their lawyer, or it can be as burdensome as requiring tax returns or bank statements.