Once the application is deployed and UAT is done, regular surveillance of application is required to check the increasing load on the application and when scalability is required, Scalability is measured in terms of throughput based on some variable. Like Number of requests/second with X users, Average response time per request, Number of records processed per second/minute, More users hitting the server, More users hitting the server; which simply means a measure of efficiency as load increases. There are more efficiency measurements that can be used, but these are common for web based systems.
The goal for a scalable application is to either maintain or improve efficiency as we deal with the load problem. In short, if the response time is taking too long, can we add another server to distribute the load evenly? This approach reduces the amount of work for one server to do, and keep the servers operating in that "sweet spot" for efficiency.
At Beryl Systems the applications are designed specifically to scale. That means we take utmost care with session data, routing requests to the right server, reducing bottlenecks that limit the ability for the application to scale either by following Vertical or Horizontal Scaling or Both as and what necessary.