A guiding principle is a recommendation that provides universal and enduring guidance to an organization, which applies in all circumstances, regardless of changes in its goals, strategies, type of work, or management structure.
https://www.bmc.com/blogs/itil-guiding-principles/#:~:text=A%20guiding%20principle%20is%20a,of%
20work%2C%20or%20management%20structure.

The purpose of the service level management practice is to set clear business-based targets for service performance, so that the delivery of a service can be properly assessed, monitored, and managed against these targets1. This practice conducts reviews to validate that services are covering the needs of the customer and to identify areas for improvement2. References: ITIL Foundation - ITIL 4 Edition, page 16; ITIL 4 - A Pocket Guide, page 37; ITIL 4 Practice Guide: Service Level Management, page 7.