Use the following options to set what happens when you select Surfaces > Untrim.
If you set Keep Original to On, the untrimmed surface is created and the original trimmed surface is retained. If you select Off, the untrimmed surface replaces the trimmed surface.
Select All to remove all trimming information from the surface. All curves-on-surface are also displayed again. All is the default.
Select Last to cancel the last trim operation. For example, if a surface had been trimmed three times, this option shows the result as if it had only been trimmed the first two times.