In PVT, Foxconn runs a small number of units through the assembly line to detect any issue that could come up during production. Quality control measures are also tested and implemented, and once passed, it will go on to be mass produced across different Apple production plants.
Citing Foxconn sources, the iPhone 6 is now in the PVT (Product Validation Test) stage, which means that the assembly line is ran at full speed in order to detect any potential issues that may hinder the assembly process. During this testing phase the factory produces a small number of iPhone 6 units for a final quality control, and if necessary, the production line is further adjusted and / or fine-tuned in order to ensure that the production systems are properly aligned. Once the PVT stage is successfully completed, the iPhone 6 will then enter mass production, just in time for its official unveiling.
Reports have been claiming now for months that Apple's iPhone 6 has entered mass production, so it is unclear whether or not which source is correct, or whether or not there was a mixup between PVT and mass production.
Many anticipate the iPhone 6 to be announced September 9, and rumors currently point to a 4.7-inch and a 5.5-inch model with an A8 processor, all new design and much more.