It's set by rebol to true if are you have Rebol/Pro enabled; otherwise it is set to false.
Rebol/Pro is inside Rebol/SDK.
REBOL/Pro has been designed for professional applications that require functions or services outside of REBOL, but do not require graphics and sound support. REBOL/Pro is very useful for server side applications and CGI scripting.
For more information look this page: http://www.rebol.com/docs/sdk/kernels.html