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
Ordinarily, each interactive user, accessing PRPC through HTTP and a browser has only a single, dedicated process, and one Thread named STANDARD.
From the portal, you can start a new Thread by clicking the Launch link in the upper portion of the navigation panel. This lets you compare two work items, in separate Internet Explorer windows.
Additional Threads are started when you use certain tools, or open additional portal windows from the Designer Studio.
Advanced capabilities
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 
13872 Contrasting PRThread objects and Java threads.
Definitions — T