Autodesk AUTOCAD 2006, Command Reference Manual, Page: 1265 / 1490

None
Turns off the U and UNDO commands and discards any UNDO command
information saved earlier in the editing session. The Undo button on the
Standard toolbar is unavailable.
The Auto, Begin, and Mark options are not available when None or One is
in effect. If you attempt to use UNDO while it is turned off, the following
prompt is displayed:
Enter an UNDO control option [All/None/One] <All>:
One
Limits UNDO to a single operation.
The Auto, Begin, and Mark options are not available when None or One is
in effect. The main prompt for the UNDO command changes to show that
only a Control option or a single step of the UNDO command is available
when the One option is in effect.
Control/<1>:
If you enter c , the previous prompt is displayed:
Enter an UNDO control option [All/None/One] <All>:
Combine
Controls whether multiple, consecutive zoom and pan commands are
combined as a single operation for undo and redo operations.
Combine zoom and pan operations? [Yes/No] <Yes>: Enter y or n or press
ENTER
Begin, End
Groups a sequence of actions into a set. After you enter the Begin option, all
subsequent actions become part of this set until you use the End option.
Entering
undo begin while a group is already active ends the current set and
begins a new one. UNDO and U treat grouped actions as a single action.
If you enter undo begin without undo end , using the Number option undoes
the specified number of commands but does not back up past the begin point.
If you want to go back to before the begin point, you must use the End option,
even if the set is empty. The same applies to the U command. A mark placed
by the Mark option disappears inside an UNDO group.
Mark, Back
Mark places a mark in the undo information. Back undoes all the work done
back to this mark. If you undo one operation at a time, you are informed when
you reach the mark.
UCS Details Dialog Box | 1241