In the Business Layer metamodel, what type of element is assigned to a Business Service?

Comprehensive and Detailed in Depth Explanation:ABusiness Interfaceis an external active structure element that provides an external point of access to one or more Business Services. In theBusiness Layer metamodel, aBusiness Serviceis assigned to aBusiness Interfacebecause interfaces define how services are exposed to external parties or other components within an enterprise.

This is confirmed in theArchiMate Specification 3.2, where aBusiness Interfaceis described as representing a point of interaction that enables access toBusiness Services.

References:

Archi User Guide​

ArchiMate 3.2 Specification