Position and size of displays can be assigned to screen and text fields via drag & drop. This way, screen and text fields can very accurately be aligned with displays, which is particularly interesting for multidisplay shows with native image display, e.g. for LED screens.
In the Media Pool click on the plus sign in front of category 
Screens and select the corresponding screen following which the screen fields and display will appear on the right.
Grab the display whose position and size you want to use and drag & drop it onto a screen or ticker field.
After confirming the message with OK, displays and screen or ticker fields will be aligned with each other.
This alignment works both ways, i.e. you can also assign position and size to a display by dragging and dropping a screen or ticker field onto it.
See also
Adding a ticker field