By now, I trust the concept of a "region" is a familiar one. If not, this might be a good place to start -> click
In brief, a region is that independent environment in which your applications run. This particular environment, when started, comes up with a set of properties that are defined in the "stanza" files. For more details on regions - how to start, stop etc, visit this link.
On AIX, each region runs as a subsystem. A subsystem is any program or process or set of programs or processes that is usually capable of operating independently or with a controlling system. A subsystem is designed as a unit to provide a designated function.
Similarly, a region running on other platforms that TXSeries supports; such as Windows, Solaris or HP, need to emulate a subsystem.
The concept of "subsystems" is unique only to AIX, hence, when a region comes up in Solaris/HP, the process "cicsstart" sets up a subsytem-like environment. It uses the "cicssrcdb" file that is present in the cics_src directory in order to access dynamic information in order to bring up the region.
That's about subsystems and the cics Region. More on cicssrcdb related blogs later!