You have to define a name or an abbreviation for the customer a/c in this field so that the customer a/c can be searched by that word or abbreviation.
Line item display and open item management are configured as a default for every customer a/c you create in SAP. For this reason you do not define the customer reconciliation account as line item display in general ledger account master data.
You can create a customer by copying an already existing customer. After creating the customer with reference to an existing customer you need to change the copied data as required by your new customer. Certain data like address of the existing customers are not copied.
Customer account group is a must to create a customer account master data. In the initial screen when you create a customer master data, you need to enter the account group. Once the customer account is created for a customer account group, the account group can not be changed. The customer a/c group is used to control the fields displayed in the customer account master. Fields can be managed as required entry, optional entry or display.
Number range for a particular customer account can be assigned internally or externally. If you define number rage as internally assigned then system internally creates a number for each customer account created in SAP, whereas external numbers are entered by the user who creates the record. Internal numbers are serially numbered by SAP and always assign the next available number in the range, where as external number depends on the user who enters it to SAP.
You can always find out the next available number by displaying the present number that SAP has internally assigned, where as in the case of external number assignment the current number can not be displayed and very difficult to have a track.
A number range defined in SAP for customer account creation can be assigned to more than one account group.