Autodesk AUTOCAD 2006, Command Reference Manual, Page: 1266 / 1490

You can place as many marks as necessary. Back moves back one mark at a
time, removing the mark. If no mark is found, Back displays the following
prompt:
This will undo everything. OK? <Y>: Enter y or n or press ENTER
Enter y to undo all commands entered in the current session. Enter n to ignore
the Back option.
When you use the Number option to undo multiple actions, UNDO stops if
it encounters a mark.
UNION
Combines selected regions or solids by addition
Solids Editing toolbar:
Modify menu: Solids Editing
➤
Union
Command line: union
Select objects: Use an object selection method and press ENTER when you finish
selecting objects
solids after UNION solids before UNION
The selection set can contain regions and solids that lie in any number of
arbitrary planes. The selection sets are divided into subsets that are joined
separately. Solids are grouped in the first subset. The first selected region and
all subsequent coplanar regions are grouped in the second set. The next region
that is not coplanar with the first region and all subsequent coplanar regions
are grouped in the third set, and so on until all regions belong to a subset.
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