From Xojo Documentation
LockHorizontal overrides LockLeft and LockRight. It allows the developer to proportionally lock a control's position to the center of its parent control (or web page).
For example, if the developer places a control in the center of the page and sets both LockHorizontal and LockVertical, the control will stay in the center of the page.
The locking properties set at design-time, so they are effectively read-only at run-time.