FND_BPM_TASK_HISTORY_B Table column Descriptions and Usage of the columns

Hi Experts,

I have build a report i.e., Payment Approval History report using BI Publisher in Fusion IC(Incentive Compensation) application as per the client requirement but client is having some questions on the columns which we are displaying in the report.

I have tried many ways to get the answers for the below questions, but i couldn't get.

Tables used in the report:

FND_BPM_TASK_B

FND_BPM_TASK_TL 

FND_BPM_TASK_HISTORY_B

Question#1  What's the difference between "Last Update Date" and "Completed Date"?  (Note : Completed Date column is fetching from the FND_BPM_TASK_HISTORY_B table)