Any one explain what happend we assign task as below"
Run immediately and At system startup,
What is the difference b/w Client Task now vs Tag Based ?
Run immediately will do just that: Run the Task once NOW
At system startup will run the task on every start of the system BEFORE the user logs in.
I don´t get the second question though. In general you are much much more flexible with TAG-based Tasks and policies.
The main thing to ask is what exactly are you trying to accomplish? Scheduled tasks are more consistent as they don't require a response from the client as a run client task now does and it is scheduled locally on the client. The schedule can be tag based or not, it just depends on your needs and structure. For updates, a scheduled task is definitely the recommendation, but other tasks might have different run requirements. It is all in planning and what you want out of the task.