A transactional exemption is a type of exemption from the general requirement that an offering of securities to the public must be registered with the SEC that's characterized by the nature of the offering. In other words, the type of offering makes it eligible for exemption from the requirement of registration. This is in contrast to an exemption based on the type of security issued, which are themselves automatically exempt from registration (e.g., securities issued by municipalities).
Startups typically rely on transactional exemptions, e.g., Rule 506(b), for their securities offerings.