There is often confusion in the use of SLASLAs and SLOSLOs. The SLA is the entire agreement that specifies what service is to be provided, how it is supported, times, locations, costs, performance, and responsibilities of the parties involved. SLOs are specific measurable characteristics of the SLA such as availability, throughput, frequency, response time, or quality. These SLOs together are meant to define the expected service between the provider and the customer and vary depending on the service's urgency, resources, and budget. SLOs provide a quantitative means to define the level of service a customer can expect from a provider<ref>{{Cite journal|last=Rastegari|first=Yousef|last2=Shams|first2=Fereidoon|date=2015-12-29|title=Optimal Decomposition of Service Level Objectives into Policy Assertions|url=http://www.hindawi.com/journals/tswj/2015/465074/|journal=The Scientific World Journal|language=en|volume=2015|pages=1–9|doi=10.1155/2015/465074|issn=2356-6140|pmc=4709918|pmid=26962544}}</ref>.
The SLO may be composed of one or more Quality[[quality of Serviceservice]] (QoS) measurements ([[service level indicator]]s, SLIs) that are combined to produce the SLO achievement value. As an example, an availability SLO may depend on multiple components, each of which may have a QoS availability measurement. The combination of QoS measures into an SLO achievement value will depend on the nature and architecture of the service.
==Examples==
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* Expression - This is the actual language that defines what the SLO will be.
Optionally aan EvaluationEvent maybe assigned to the SLO, whichan EvaluationEvent dictates the measure by with the SLO will be checked to see if it's meeting the Expression.
In <ref name=":0" /> the authors define the SLO as "the quality of service aspect of the agreement. Syntactically, it is an assertion over the terms of the agreement as well as such qualities as date and time".
SLOs should generally be specified in terms of an achievement value or service level, a target measurement, a measurement period, and where and how they're are measured. As an example, "90% of calls to the helpdesk should be answered in less than 20 seconds measured over a one-month period as reported by the [[Automatic call distributor|ACD system]]". Results can be reported as a percent of time that the target answer time was achieved, and then compared to the desired service level (90%).
