It happened to me several times I had to debug how SLAs are started/paused/cancelled or how the elapsed time or planned end time is calculated. Sometimes they were different requirements to modify something here and there in the way how SLA engine works in ServiceNow.
Everytime I had hard times to fully understand how all the SLA related scripts are linked together and what are the main responsibilities of each of that script. To make it simpler for me next time I created following diagram that shows the relations of main SLA scripts like "TaskSLAController", "SLACalculatorNG", "SLAConditionBase", "TaskSLA" and so on.
Hopefully this will help you to find quickly the place of particular SLA implementation in your ServiceNow instances.
