This is used to talk about position of the control and the container.
When we talk about the position of the control (for example ControlPosition for TUIRectangularControl.Align), it determines which border of the control to align.
When we talk about the position of the container (for example ContainerPosition for TUIRectangularControl.Align), this specifies the container border.
In most cases you use equal both control and container borders. For example, both ControlPosition and ContainerPosition are usually equal for TUIControlPos.Align call. This allows to align left control edge to left container edge, or right control edge to right container edge, or to center control within the container — which is the most common usage.
If both are prLeft, then X specifies position of left control border relative to left container border. X should be >= 0 if you want to see the control completely within the container.
If both are prMiddle, then X (most often just 0) specifies the shift between container middle to control middle. If X is zero, then control is just in the middle of the container.
If both are prRight, then X specifies position of right control border relative to right container border. X should be <= 0 if you want to see the control completely within the container.