It is used to define account class in FI/GL, for example 
intercompany customers and external customers belong to 
different account group

Customer master group is a controlling element for customer 
master. It contols overall views for CM i.e. which data 
should appear on not in CM. This is also used for level of 
reporting

Accoutn group is used in three areas,

first, through account group you can control the which are 
the field in customer master is mandatory i.e. you can 
switch off and on.

secondly, you can control the no. ranges of customer master.

thirdly, it is used in partner determination process.

Customer account group is controlled  basic data of 
customer in order such as general data, company data and 
sales and display besic setting of customer in order

Account Group is define it's a sold to party or ship to
party,or bill to party, or payer and also control number
ranges of customer's

Account Group controls :
1)Number range of the customers
2)Whether the numbering will be internal or externally 
assigned
3)what field will be displayed in the customer master 
record and which one will be 
suppressed/hidden/optional/mandatory.

The configuration of account group is done in 
IMG>FI>Account Receivables and Payables.

Customer Account Grp. is the controlling element of the CMR.
It controls the following:
1) Number Ranges
2) Whether One time a/c
3) Field Selection
4) Text Determination Procedures
5) Partner Determination Procedures
6) O/p Detremination 
7) Customer Pricing Procedure
8) Other options can be enabled such as the type of cust. 
For Eg. Competitor or Sold to Party..etc..

customer account group is one of the important requist for 
creating customer master and another one is partner 
determinations .after determining pdp we can assign the pdp 
to account groups .

It is a structure of customer master record.what ever the 
things is mandatory for particular customers we will select 
that one when ever we are doing this mastre data we should 
enter