Dear Friend,
Im not sure if my assumption for the answer is correct.

In customisation you customise with the existing set up.

For eg: extend the existing sales office to other Sales org 
as well

In configuration you create a unit itself
For eg: Create a new sales org.

In short customisation is part of configuration.

Im assuming the answer by the process we follow,
we create a configuration document only if in case there 
was a new additonal unit created.
In case if there is only assignment or changes with the 
existing data we create only Scoping document and not 
Configuration document

Correct me if I am wrong.

Thanks and regards
Siva

Hai,
   There is a lot of misconception on Customisation and 
Configuration, according to my knowledge both are same.

Configuration is done based on the NEEDS of business 
requirements .For Example : Adding fields in dropdown list .

Customization is where we ADAPT the based on business 
requirements. Cusomization once done is difficult to 
modify .

configuration is arranging or mapping business scenario's 
provided by the sap standard  systematic approach .where as 
customization arises when there is gap in between the sap 
standard and business requirement,at that time we need to 
custom the sap as per requirement .For this sap has 
provided us uerexits, routines, and abap programme editors 
etc., are give to use for customization

Customization - word itself explains us...i.e., using the
standard system available making few changes as per client
requirement.

Configuration - means... as per business(client) requirement
 developing / creating a new feature or new structure. 

In other words we do not configure new sales doc type...
what we do is customize i.e., the available standard doc
with few changes as per client needs.

CONFIGURATION: we will configure the system to meet the
needs of your business by using the existing data. 

CUSTOMIZING: we will customize or adapt the system to your
business requirements, which is the process of mapping SAP
to your business process.

Configuration vs. Customization:
 
When considering enterprise software of any type, it is
important to understand the difference between configuration
and customization. The crux of the difference is
“complexity”. Configuration uses the inherent flexibility of
the enterprise software to add fields, change field names,
modify drop-down lists, or add buttons. Configurations are
made using powerful built-in tool sets. Customization on the
other hand, involves code changes to create functionality
that is not available through configuration. Customization
can be costly and can complicate future upgrades to the
software because the code changes may not easily migrate to
the new version. Wherever possible, companies should avoid
customization by using configuration to meet their goals.
Companies also should understand their vendor's particular
terminology with regard to this issue since words like
"modifications" or "extensions" often mean different things
to different vendors. 

Regards,
Prasad JRN