Assembly vs. Subassembly in Epicor: What’s the Difference?

When working in Epicor ERP, the terms Assembly and Subassembly often appear throughout Bills of Materials (BOMs) and production Jobs. While they may sound similar, they play distinct roles in defining a product’s manufacturing structure. Understanding how these two elements work together is key to improving accuracy in production planning, costing, and inventory control.

What Is an Assembly?

An Assembly is a primary-level manufactured unit within a Job. It represents a product or component that can stand on its own and is fully defined by its materials and operations.

An Assembly in Epicor:

• Has its own Bill of Materials (BOM) listing the components required for production.

• Contains all operations, resources, and routing steps needed to manufacture it.

• Can represent a finished product or an independent component used in another build.

Example:

An Assembly called Rim-Tire might include:

• 1 Rim

• 1 Tire

Once completed, the Rim-Tire can be stocked in inventory or used later as part of a larger build.

What Is a Subassembly?

A Subassembly is an Assembly that becomes part of another, higher-level Assembly. It exists within the hierarchy of a more complex product, serving as a building block of the final item.

Example:

If you are manufacturing a Car, its BOM may include:

• Engine (Subassembly)

• Door (Subassembly)

• Rim-Tire (Subassembly)

In this case, Rim-Tire is no longer a stand-alone Assembly—it functions as a Subassembly within the Car Assembly.

Key Differences Between Assembly and Subassembly

Aspect	Assembly	Subassembly
Hierarchy Level	Can exist independently or as a top-level item	Always part of a higher-level Assembly
Bill of Materials (BOM)	Has its own BOM	Its BOM feeds into the parent Assembly
Example	Rim-Tire (with Rim and Tire)	Rim-Tire within the Car

Applying Assembly Hierarchies in Practice

Epicor ERP’s flexibility allows manufacturers to structure Assemblies and Subassemblies in many ways. Understanding how to use these levels strategically can improve operational performance and reduce errors.

• Production Scheduling:

  Proper hierarchy setup helps Epicor determine build order. Subassemblies are scheduled first so the parent Assembly can be completed without delays.

  
  

• Material Requirements Planning (MRP):

  When Assemblies and Subassemblies are defined correctly, MRP can accurately calculate material demand and prevent shortages or overproduction.

• Inventory Management:

  Defining whether a Subassembly is Make Direct or Make to Stock affects how materials are consumed and replenished. This decision impacts both cost tracking and production efficiency.

Best Practices for Managing Assemblies in Epicor

1. Reuse Subassemblies Across Products

   If a Subassembly (like Rim-Tire) is used in multiple finished goods, standardize it. This simplifies BOM maintenance and ensures consistent costing across Jobs.

   
   

2. Validate Cost Roll-Ups Regularly

   Review Epicor’s cost roll-up reports to confirm that Subassembly costs accurately reflect material, labor, and overhead. Misaligned cost data can distort profitability analysis.

   
   

3. Keep BOMs Clean and Structured

   Avoid excessive nesting or unnecessary levels. Too many Subassemblies can overcomplicate scheduling, while too few may limit flexibility.

   
   

4. Leverage Templates and Revision Control

   Epicor allows you to version Assemblies and Subassemblies. Using templates and revision tracking reduces manual errors and helps maintain traceability.

Common Mistakes to Avoid

• Creating every component as a top-level Assembly instead of nesting related parts.

• Forgetting to define Make Direct vs. Make to Stock logic.

• Skipping cost roll-up verification, leading to inaccurate costing.

• Not linking shared Subassemblies between similar products, causing redundant work.

Final Thoughts

In Epicor ERP, every manufactured component with its own materials and operations qualifies as an Assembly. When that same component becomes part of a larger build, it functions as a Subassembly. Understanding and applying this distinction correctly gives manufacturers the structure needed to plan efficiently, track costs accurately, and maintain control across multiple production levels.

When Assemblies and Subassemblies are organized strategically, teams gain visibility into each stage of production, reduce rework, and make better planning decisions—all foundational elements of an effective ERP strategy.