What is Ultimate Thread Group?

The ultimate thread group is highly customizable and unlike the arrivals, or freeform and concurrency thread groups, this one kills the active threads after the set time has expired.
The start threads count field: You set the target thread concurrency.
The initial delay: Set how much time you wait before starting to execute the threads from that specific row.
The startup time: The ramp up time for all the threads set in the thread count. For example, for 20 threads and a start time of 20 seconds, you would have 1 new thread firing up every 1 second.
The hold load for field: This is where you set the duration to maintain the defined number of concurrent users and is totally independent from the ramp-up time.
The shutdown time:  The time interval for killing all the active threads.