'Class E' is a broad category of commercial, business and service uses. It was introduced by the Business and Planning Act 2020.
Class E effectively amalgamates the former Class A1 (retail), Class A2 (financial and professional services), A3 (restaurants/cafes), B1 (offices) along with health/medical uses, creches, nurseries (all formerly D1 uses) and indoor sports/recreation (formerly D2 use). Permission is not required to change between any of the uses within Class E.
The only retail uses not included in Class E are small shops under 280 square metres, selling 'essential' goods and not located within 1km of other similar shops. These shops are in Class F2.
The full regulations can be accessed here: https://www.legislation.gov.uk/uksi/2020/757/made