UI position
There are two types of positions - Anchor and Offset. Both have Min and Max. Min value is the bottom left corner position, and Max is the top right one. Both Min and Max have X and Y values and are such string: "X Y".
Anchors are numbers that represent width and height in percents (%) from 0.0 (0%) to 1.0 (100%).
Offsets are accurate values that are width and height in pixels relative to 1280x720. Example: You made a 100x100 box for 1280x720 and if you switch to 2560x1440, it will be 200x200 pixels.
Anchors are applied before offsets. If you want offsets to be relative to the screen center, make all anchors 0.5 0.5 so that both corners are in the center. Then to make a 100x100 box you should use offsets -50 -50 for bottom left corner and 50 50 for top right one.