“Combine” will ignore all the selected objects’ curve properties and combine those objects into a new curve combination with new curve properties. Same as other objects, this new combination can be selected, copied, pasted and set properties.
“Uncombine” will get the combination back to the curve objects separately. The objects after “Uncombine” may be different from those before “Combine”. “Undo” should be used to restore the original objects.
The shortcut keys of “Combine” and “Uncombine” respectively are: Ctrl+L and Ctrl+K.