Hi @umeshbhingare
When a task record changes, typically a new SLA may be attached, with a new set of timing information. This is useful if you are re-assigning an incident to another group and want to attach a new SLA record with new timing information.
However, you may want to retain time information for the task in specific situations. For example, an incident is raised with a priority of 3 - Moderate and the priority changes to 1 - Critical after 3 hours. A priority 1 SLA is attached to the incident at that time. You can use retroactive start to ensure this SLA timing is adjusted retroactively to count from when the incident was first created, rather than from when the incident's priority changed. This reflects the actual time the user contacted you.
