Attack plot method using the attack disk

[Captain_AJ]

All .. I am a bit confused about a method called the attack plot method .. I have watched over and over many tutorials on you tube about the attack disk and how to use, However since I am using real navigation without map updates the very part were I am confused is , why is it possible to mark an x any were on the map and have that represent the position of your u boat .. is there a place were this is explained in a step by step read me that supports the reason why you randomingly can mark the location of your u boat in relation to your target ?

what Iam looking for is a explantion of the method in detail thanks for your help