Hi Olivia,
That is a fairly open question because it tends to depend on the application container/runtime environment.
Here is a previous community post on using Transaction Trace which may help your understanding as you just asked about TT in your latest post
Spot the Application Performance Bottleneck with APM Transaction Tracing
Also the wiki mentions it for the Average Response Time & Concurrent Invocations metrics:
BlamePoint Metrics - CA Application Performance Management - 10.5 - CA Technologies Documentation
Hope that helps to start with.
Please let us know if you have more specific questions in due course
Regards,
Lynn