Products Required on the Work Type is the appropriate Field Service design. A Work Type acts as a reusable template for recurring service. Adding standard parts to its Products Required related list lets those requirements flow to Work Orders or Work Order Line Items when the Work Type is applied.
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. Product Items show stock at a Location, and custom Flow or layout fields do not replace the standard reusable Products Required model.
This approach keeps the implementation aligned with the standard Field Service data model and avoids unnecessary customization. Study Guide reference: Managing Inventory - Work Types; Products Required; inventory planning.