Allocating Threads to a CPU
By default, each instance of SQL Server starts each thread. If affinity has been enabled, the operating system assigns each thread to a specific CPU. The operating system distributes threads from instances of SQL Server among the microprocessors, or CPUs on a computer based on load. Sometimes, the operating system can also move a thread from one CPU with heavy usage to another CPU. In contrast, the SQL Server Database Engine assigns worker threads to schedulers that distribute the threads evenly among the CPUs.
The affinity mask option is set by using ALTER SERVER CONFIGURATION. When the affinity mask is not set, the instance of SQL Server allocates worker threads evenly among the schedulers that have not been masked off.