Within Settings - Inventory - Manage Categories you can manage the visibility of inventory-related fields within different parts of the system.
Enable: Enables the field within customer asset records
Grids: Enables the field within the asset grid (customer record - Assets tab)
Data Capture: Enables the field in Data Capture, accessed from a sales order record
Label: Allows you to present a different name for this field for this particular product category
Note the tabs at the top, which allow you to show different fields for services and products.
Your instance's own custom fields are also displayed on their own tabs.