We have changed the terminology between PVP 2 and PVP 3 in regards to Targets and Target Sets. In PVP 2, Target Sets were an optional subset of Targets. In PVP 3 it is the opposite; Targets are a required subset of Target Sets.
Target
A Target is an area of the Canvas where PVP will play content. It is defined by an overall shape (most often a rectangle, but it can be an oval or a custom shape) and its primary attributes are its Position and Size. You can create a Target at any location, however only parts of a Target that overlap a Screen will render on your physical devices.
For example: You want a piece of media to play only in the top left of one of your devices, so you create a Target that covers the top left of that Output.
Target Set
A group of Targets that will all play the same media is called a Target Set. Note that there is no limit to the number of Targets within a Target Set.
For example: You want the same media to play on two different Screens at the same time, so you create one Target Set that contains two Targets (one on each Screen). You want different media to play on a third Screen, therefore you create a second Target Set which contains one Target on the third Screen.
For more information on Targets and Target Sets you can check out the PVP 3 tutorials page, and also take a look at the user manual.