They're not trading in securities, although they would deal in securities as a part of the underwriting process.

他们不买卖证券，虽然在经营证券,是他们承销的一个环节。

耶鲁公开课 - 金融市场课程节选

Public securities are securities that the SEC will accept as suitable for the general public and private securities are securities that have not been vetted by the SEC for the general public.

公共券商是指那些经证交会审核通过,适于面向大众经营证券买卖的证券公司,而私募券商是指那些未经,证交会审查,面向大众的证券公司

耶鲁公开课 - 金融市场课程节选