For this scenario, Salesforce Field Service should use Products and Price Books. The consumed part is a Product, and pricing is supplied through Price Book Entries so the Product Consumed record can carry list price, unit price, discounts, and billing context.
Using the standard inventory objects keeps quantity, location, serial number, price, and service history aligned. Custom fields or improvised notes may capture text, but they do not participate in the native stock, fulfillment, consumption, and reporting processes that Field Service provides.
The alternatives address different parts of the Field Service model. Assets identify installed customer equipment, while Opportunity Line Items belong to the sales process and custom objects are unnecessary for standard Field Service part pricing.
The standard inventory model also keeps planned demand, stock position, movement, actual consumption, and pricing available for reporting without duplicating the same information in custom records.
This approach keeps the implementation aligned with the standard Field Service data model and avoids unnecessary customization. Study Guide reference: Managing Inventory — Products Consumed; Products; Price Books; pricing.
