Thread
A PegaRULES Thread object is a named context of clipboard pages. It does not correspond to a UNIX or Windows operating system thread. Most processing for a requestor is single-threaded. The pxThread page is a named top-level clipboard page sometimes referred to as the thread page. The class for the page is Code-Pega-Thread.
Starting a new Thread with Launch
Initially, each interactive user, accessing PRPC through HTTP and a browser has only a single, dedicated process, and one Thread named STANDARD.
As you interact with PRPC , you start new Threads in a variety of ways. These include opening additional portals, displaying portal tabs (creating rules or opening landing pages), or creating work items in a process or a wizard.
Advanced capabilities
R-8914 03-04 MIRAT 
If you include the property pyThreadUse in a URL sent with a Submit button (that is, an HTTP Post operation), PRPC starts a second Thread with that name. The url JSP tag supports this capability.
When designing and implementing multiThread interactions for a browser-based user session, as a best practice, have each Thread that uses the Show-HTML method direct its HTML to a specific, named HTML frame on the browser window, to avoid confusion.
The PublicAPI facilities supporting Threads are part of this package:
com.pega.pegarules.pub.context
For more explanation about Threads, see PDN article Contrasting PRThread objects and Java threads.
Definitions