Grouping some assets in to one group ie., for ex; Plant &
Machinery is one group what ever assets comes under p&m will
will group into that p&m.

Asset group means, we fibercate the total assets.

Asset: Furniture & fittings, Asset:plant&mach-Ele.Equipment
asset:plant&machinery-etc., The group is easy to identify 
the assets and easy to set the Depreciation key. for 
eg.Funiture & Fittings-Dep.18% is one group. some furniture 
like that cinema shed which 100% dep. So we have to create 

Fur&Fitt 18%
Fur&Fitt 100%
It will be easy to create asset under the relevant group 
system automatically calculate the depreciation based on 
the depreciation key.

If you have more doubt ih this asset, ask agains

1).  The  concept of  group  assets  becomes  necessary  
when  u  need  yo  carry  out  deprecaition  at  a  group  
level  for some  special  purposes  like  tax reporting,  
this  can  have any number  of  sub  assets denoted  by  
asset  sub-  number.
2).Sap  way  of  depreciation is  always  idividual  asset 
level.  if  we  ned  to  calculate  depreciation  at  
group  level  we need  to  group  assets.for this  we  
need  special d epreication  area  for  calculate  group  
depreiation.