A service level agreement (SLA) is a documented agreement between a service provider and a customer that identifies both services required and the expected level of service1. A necessity to a successful SLA is that the language and terms used in the SLA should be commonly understood by all parties, to avoid ambiguity, confusion, and disputes2. The other statements are not true because:

Base the SLA on system-based metrics that are useful to the service provider: The SLA should be based on customer-based metrics that are meaningful to the customer and reflect the value of the service2.

In order to promote consistent service, they should be carried forward, unchanged, from one year to the next: The SLA should be reviewed and updated regularly to reflect changing business needs, customer expectations, and service performance2.

Vague targets, such as those related to user experience should be avoided: The SLA should include both quantitative and qualitative targets, such as those related to user experience, satisfaction, and perception, as well as availability, reliability, and security2. References: ITIL Foundation - ITIL 4 Edition, page 16; ITIL® 4 – A Pocket Guide, page 37; ITIL® 4 Practice Guide: Service Level Management, page 8.