Hello!
When it comes to SLAs for service availability, anything less than their successful uptime percentages would be in need of improvement. Technically anything other than successful could fall under "Improvement Needed".
In our practice, if the agreement only states acceptable and unacceptable ranges, we will leave the Needs Improvement section blank. However, if the agreement has multiple levels of availability ranges with associated credits, I add anything in between successful and unacceptable in the Needs Improvement section.
Example: Acceptable being 99.95%, no credits (Successful Performance); Less than 99.95 but greater than 95.0 would receive a 10% statement credit (Improvement Needed). Less than 95.0% is unacceptable, 25% credit or option to terminate (Unacceptable Performance).
Hope this is helpful! I would love to hear if anyone else has any suggestions or can share how they handle this situation with their SLA monitoring.