6.16.3 Closing a Tab Sheet
If you want to allow the user to close the tab sheet, use its DockCloseButton property to specify whether a Close button is displayed in the tab sheet. This property can be set to True or False:
.
When a tab sheet has been closed, its Hidden property is set to True.
Use this property with the ShowSheet and HideSheet methods to determine displaying or redisplaying of a tab sheet.
Note that the Hidden property will be True if a tab sheet has been undocked and the User closes the undock form.