Working Set directly address thrashing problem in which is the set of pages referenced by the process during the most recent w page references. If we assume that T is sum of sizes of the working sets, then if T is less than total number of frames then operating system can allow more active processes, else operating system must suspend some active processes, otherwise thrashing may occurs. However, if the T is equal to set window size then there is fixed number of page references occurs.
Locality of Reference:
The locality of reference or principle of locality is a phenomenon that the same values or locations are accesses frequently. There are many types of locality of references. However, there is two basic types of locality of reference: Temporal locality in which same data can be accessed in accessed again in the near future. Spatial locality in which same place/block can be accessed in accessed again in the near future.