Certain aspects of SCL’s implementation may seem counter-intuitive. As with most engineering endeavors, a short history can help explain how SCL arrived at where it is today.
SCL has been a part of the MPLAB IDE Simulator for some time, dating back to at least 2005. Unfortunately, the Microchip engineer who developed SCL left the company shortly after completing the task. This left the MPLAB IDE Simulator team somewhat reticent to take on maintenance of the poorly understood, and somewhat complex, SCL software. So the MPLAB IDE Simulator team built a front-end GUI to SCL called Stimulus. The idea was that Stimulus would only ever generate SCL code that the team was confident would work. Unfortunately, many of the SCL capabilities are not exposed by the Stimulus GUI.
The MPLAB X project, which ported the MPLAB IDE to operating systems other than Windows, required a complete rewrite of the entire Simulator, including the SCL portion. The rewrite effort provided an excellent opportunity to learn and document the SCL software. This manual is part of that effort.
Despite being somewhat hidden by the Stimulus GUI, MPLAB IDE Simulator users quickly discovered the underlying SCL language and began writing their own SCL processes. The Microchip Forums contain many detailed discussions of SCL, including a fairly detailed user deconstruction of the SCL language and an SCL code repository.