This picture shows the relationship between UML processes and host processes. Each UML process has a host process. The host process is used solely as an execution context. It will have completely different attributes from the UML process, including different name, different uid, and different pid.
It also shows the tracing thread, whose job it is to trace the system calls of all the other UML threads so that they may be intercepted, nullified, and executed in the context of UML.