Scalability testing is a subtype of performance test where 
performance requirements for response time, throughput, 
and/or utilization are tested as load on the SUT is 
increased over time.

as a part of scalability testing we test the expandability 
of the application.
in scalability we test 1.Applicaation scalability
                       2.Performance scalability
Application scalability: to test the possibility of 
implementing new features in the system or updating the 
existing features of the system. with the help of design 
doc we do this testing

Performance scalability:
 to test how the s/w perform when it is subjected to 
varying loads to measure and to evaluate the the 
performance behaviour and the ability fo the s/w to 
continue to function properly under different workloads.

--> to check the comfort level of an application in terms 
of user load. and user experience and system tolerance 
levels
--> the point within an application that when subjected to 
increasing workload begin to degrade in terms of end user 
experience and system tolerance
--> response time
    execution time 
    system resource utilization
    network delays
   
--> stress testing