in the definition you will read:
1) The panel border defines the area of the board that can have trace and plane data. The PanelBorder object is valid in xPCB FabLink and xPCB Drawing Editor applications.
2) The panel outline represents all elements of a panel with the exception of solder palettes and sky bars (which may be outside the outline). The PanelOutline object is valid in xPCB FabLink and xPCB Drawing Editor applications.
Hope this helps
Some other differences are: - PanelBorder is taken from system on calculation basis. Means how many single PCB boards could be placed inside. - PanelBorder could not be extracted to ODB++. - PanelOutline must be as big as needed to get all (wished) informations/data to xPCB Drawing Editor. The behavior is similar as the manufacturing outline in xPCB Xpedition. - PanelOutline will be extracted to ODB++ and will re-named to 'odb_panel_outline'. It will be your Profile outline in ValorNPI tool set.