To access the Customer Item Settings open the Settings module, and locate the Customer Items section. Within this section are Customer Item Settings:

This allows you to define key settings that will impact Customer Items.

• Next Item # - this lets you set the next item number used when a Customer item is created
• Require user confirmation of customer item details before posting invoice - 
• Require unique serial numbers within each customer item type – This creates a requirement for all newly created Customer Items to have a unique serial number assigned to them.