From Xojo Documentation
LockVertical overrides LockTop and LockBottom. It allows the developer to proportionally lock a control's position to keep it centered within the 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.