P3X-042 was the planet from which the System Lord Yu reigned over his domain. Some time in 2001/2002, Jacob Carter/Selmak went undercover on this planet by impersonating a minor Goa'uld following Yu. When the Tok'ra got information about a large System Lord summit in the Hasara system, the Tok'ra informed Stargate Command to get their help. Selmak and Dr. Daniel Jackson infiltrated the planet and Jackson was able to convince Yu that he was his personal Lo'taur by using the Reol chemical on him. (SG1: "Summit")
Several weeks later, Teal'c, while working for the rebel Jaffa K'tano, went on a mission for him against Yu but was captured and brought before the System lord on his homeworld. After informing Teal'c about K'tano's true intentions, Yu let Teal'c go. (SG1: "The Warrior")
Behind the scenes
Yu's temple is very similar to the layout of Potala Palace in Lhasa, Tibet.