You are a solution architect and your client has recently completed a merger and acquisition.

 SAP Cloud Identity Services - Identity Authentication enables secure single sign-on (SSO) and user authentication for cloud-based applications.

 It is appropriate for enabling access to the custom application without adding users to the global directory. This ensures secure access while maintaining compliance with business and legal constraints.

 The service is designed for scenarios where temporary access or isolated authentication is required, as depicted in the provided diagram.(Reference: SAP Cloud Identity Services)