The following should be true:
1. All Tasks set as "Mandatory" must be marked completed before an Incident can be closed.
2. All Incident Fields present on a Mandatory Task, must be updated/selected before that Task can be marked complete. Then all Mandatory Tasks must be marked complete, before an Incident can be closed.
3. All Tasks set as "Optional" do NOT have to be marked completed before an Incident can be closed.
This ensures that Analysis is performed as directed in a Task, and that product of that Analysis be captured in the appropriate Incident Fields present on that Task.
Tasks do not necessarily need to be completed in order (serially in workflows they must be completed in order), so if a Field/Analysis cannot be performed at that time, an analyst is free to work on other present tasks (the designer/lead may wish to move Tasks in a more appropriate Phase/Phase Order based on feedback.)