Some controls need to be “attached” to the another in order to work. For example, many controls can have context menu. But context menu is substitute control, so this is placed separately on the form. To let Koda know that this control should use certain context menu, you should select ContextMenu control from the “ContextMenu” property. In dropdown list you see only controls, that can be attached to this property.
If you choose control that already attached to another control, this will be reattached to new control.
Usually control can be attached only to one control. The only exception - ImageList, since it can be used in different controls at once.