Airspace where IFR and SVFR flights require ATC clearance, and, in return, are provided with separation services by ATC, but VFR flights do not, and are not.
Class E airspace, one of the six or seven airspace-classes, is the least restrictive of the five classes of controlled-airspace. instrument-flight-rules (IFR) and Special Visual Flight Rules (SVFR) flights require clearance from air-traffic-control in order to operate in class E airspace, but, in return, ATC is responsible for maintaining a safe amount of separation between different IFR and SVFR flights. visual-flight-rules (VFR) flights do not need ATC clearance to operate in class E airspace, and are thus responsible for maintaining their own separation by "seeing and avoiding" nearby flights. ATC issues traffic advisories to all flights to notify them if another flight could potentially pass dangerously close to them, but the duty to see and avoid said traffic is on the pilots.