ThreadPriority is a TIdThreadPriority property that represents the default thread priority assigned to threads created in the thread-based scheduler. 

The default value for ThreadPriority is tpNormal, as assigned during initialization of the component instance. Assign a value from the TIdThreadPriority enumeration to the property to alter the priority given to the thread during pre-emptive scheduling in the operating system. 

ThreadPriority is used in NewThread when initializing the TIdThreadWithTask instance for the yarn.

NewThread 

TIdThreadWithTask 

TIdThreadPriority 

SetThreadPriority