Note
See the following video to learn more about the purpose and structure of a bill of material in SAP S/4HANA Service:Creating a BOM

You can create all categories of BOM with or without using a template. If you use a template, the following rules apply for each category:
- To create a functional location BOM or equipment BOM, you can use material BOMs, equipment BOMs, and functional location BOMs as a template.
- To create a material BOM, you can only use material BOMs as a template.
Note
As of SAP S/4HANA 2021, the following feature is available:
Automatic proposal of the item category when adding components to maintenance bills of material based on the plant and material type of the added component.
BOM Assignment

A BOM can be linked with a technical object in two ways: direct assignment and indirect assignment.
For direct assignment, a BOM is created directly for the object (functional location or equipment).
For indirect assignment, you can assign a BOM to master records for technical objects using a material master record. Create the assignment in the master record for the technical object by entering the material number in the Construction type field. The BOM is dependent on the material, not on the technical object. Indirect assignment is advisable if several objects exist for the same construction types. You do not have to create an equipment BOM for each individual object – all objects of the same construction type use the same material BOM.
BOM Structure

The data that you maintain in the BOM header is valid for the entire BOM with its components (BOM items or sub-items).
The components of a technical object are located in the BOM items. Item data is only valid for a particular item within the BOM.
BOM Items in Service Order and Maintenance Order

You can schedule BOM components from the reference object (materials), which are required to execute the task, for each operation in the service order. This is called component assignment.
The materials that you schedule for the maintenance order are reserved in the warehouse if they are kept in stock. The automatic creation of the reservation is triggered via the item category.
The process flow is different if you schedule non-stock materials. In this case, ordering is initiated using item category, which results in the automatic creation of a purchase requisition.
In Service with Advanced Execution, you use the following item categories assigned in the component list of the maintenance order:- L (stock material)
- N (non-stock material)
Item Status

In each company, BOM usage (in the Customizing for Production) defines the allowed entries for the item status. Various BOM item indicators are combined under the heading "item status", and they include relevant indicators for engineering/design, production, costing, and so on.
The item status indicators control the process flow in subsequent areas of work and the selection of items in the BOM explosion. They require, enable, or exclude processing in these areas. If processing is supported in an enterprise area, you can maintain area-specific data for the items. For example, only items relevant for production are copied to the production order.
To maintain structure elements or items relevant for maintenance for a functional location BOM or equipment BOM, you must select a BOM usage that supports items relevant for maintenance. BOM maintenance for the different areas (for example, engineering/design, production) is performed separately for each usage. If you create several BOM with different uses for one material, the system saves the BOM for each usage with their own internal number.
Maintenance Assemblies
Assemblies act as structure nodes within a BOM and combine spare parts relevant for maintenance.

Maintenance assemblies are materials of material type IBAU. The material master record only contains basic data and classification data, so inventory management is not possible for maintenance assemblies.
Use maintenance assemblies as structure elements in a BOM if you want to complete either of the following tasks:
Combine similar materials under one node.
Track the costs in the Plant Maintenance Information System, but you do not require inventory management.
However, you also have the option of selecting a material of another material type as a maintenance assembly.
The Maintenance Assembly Indicator is set on the Status/Long Text of the BOM items screen and can only be maintained for items relevant for maintenance. During the processing of maintenance tasks, items marked as a maintenance assembly are displayed as structure elements of an operational system. They can be used to describe an item from an operational system in greater detail (for example, possible location of damage).