The Process Buffer is essentially an asynchronous shell running inside SlickEdit®. The shell runs in a normal edit window in which text can be cut, copied, and pasted to and from. It is called the concurrent process buffer because commands can be run concurrently inside it. When running a compile or build, the command is run inside the .process buffer by default. For example, if a long compile or grep command is running inside the .process buffer, then editing can continue.
To start the .process buffer manually, just go to Build > Show Build.