Customer/Prospect Header Fields
Customer header fields are located above the folder tabs of the Maintain Customers/Prospects window. This is where you enter lookup information about the customer such as the customer ID, name, and status.
Customer ID: Use this field
to create a unique ID for this customer. If you leave the ID field blank, Sage 50 will create an ID based on the Name you enter (provided this option is selected in Global Options.) When entering transactions, you
can enter the ID or the Name to identify the customer. You can sort and filter reports and lookup lists
using the Customer ID. 
More information
Back and Next: Use these buttons to navigate through the list of existing customer records by ID. Select the Back (left arrow) button to see the previous record in the list; select the Next (right arrow) button to see the next record in the list.
Name: Enter up to 39 alphanumeric characters for a new customer's name.
Prospect: If you check this box, this company or person is not included on any customer reports. If you generate an invoice for the prospect or clear the check box, the prospect becomes a regular customer and is included on customer reports.
Inactive:Select this checkbox to make this customer inactive. Inactive customers can be purged if they don't have any outstanding transactions. You can still create transactions for inactive customers, but you'll get warnings when you do so. You can exclude inactive records from lists. This setting is in Global Options. You can use the Write Letters functionality to send letters specifically to your inactive customers to try to regain their business.
Tip: If you want to make multiple customers inactive, you can go to the Data Maintenance section of the System Navigation Center and click Make Records Inactive.