Task Audit Tab versus Task Assignment Audit Tab

Which activities trigger an update to the Task Audit Tab but not to the Assignment Audit Tab and vice versa?

For example, when a task gets updated with Actual Effort, an audit entry is made on the Audit Tab but not on the Assignment Audit tab even though there is a column for Actual Effort in both tabs.

What is the intended purpose of each tab? We upgraded to 12.1.3 and did not have the Assignment Audit tab in 11.5.10. Can anyone point me to documentation about this or explain it?

Thanks,

Cheryl