In PVP 3 you will see references to "outputs" and also references to "screens". While these are relatively similar in function, there is an important distinction to be made between them.
A screen is the digital representation of your physical devices inside of the workspace. The primary property for each screen is its size (Height and Width) and its position (X and Y).
For example: You are setting up PVP to control two TVs, so you add two screens to your workspace, one for each physical device you will be sending a signal to.
The output is the property of a screen which controls how the media is actually sent to the physical device. Its primary attribute is its size (resolution) which will generally be the full resolution of the physical device, however it doesn't have to be.
For example: You are setting up two TVs, one will be using 1920x1080 and the other 1280x720. You would set up one screen to use an output size of 1920x1080 and another screen to use an output size of 1280x720.
The output target is an optional setting where you can control where the output will appear inside of (that is as a subset of) the screen. The primary properties of an output target are its size (resolution) and its position.
For example: Often times LED walls use only a small portion of your actual output, such as an LED wall that is 300 pixels by 300 pixels but whose processor wants a 1920x1080 signal from the computer. In this case you would set up your screen to be whichever relative size you need it to be to match the other screens (or use 300x300 if relative size is not a factor), the output would be 1920x1080 (because that is what the computer is outputting to the LED processor), and the output target would be 300x300 because that is the actual resolution of the physical wall.
For more information on how to create screens and change output settings, you can check out the PVP 3 tutorials page and/or take a look at the user manual.