> ## Documentation Index
> Fetch the complete documentation index at: https://docs.firstresonance.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Alternates and substitutes

> How to configure interchangeable parts on the mBOM, and how operators install substitutes during runs without filing deviations.

A substitute is an interchangeable part configured against a build requirement on the mBOM. See [Bills of materials overview](/build-hardware/bills-of-materials) for how substitutes sit on the line. When a substitute is approved, operators can scan it during install in place of the primary and ION accepts it without flagging a deviation. This page covers configuring substitutes and how they behave at install time.

A different revision of the same part is a revision-interchangeability question, not a substitute. Set that on the part record instead. See [Enable part interchangeability](/administration/supply-chain-settings/enable-part-interchangeability).

## Configure a substitute

From the **mBOM** card on the part page, with the version in **Draft**:

1. Find the build requirement's row.
2. Open the **Substitutes** dropdown in that row.
3. Pick one or more substitute parts. Selections save as you make them.

To remove a substitute, reopen the dropdown and deselect it. You can add or remove substitutes without revising the mBOM as a whole, since they're metadata on the line. Significant substitution changes, such as flipping the primary, typically warrant a new mBOM revision.

## How substitutes behave during install

When an operator installs against a build requirement at run time:

1. The Part Installs (aBOM) dialog shows the **primary part** prominently.
2. The configured substitutes are listed under the primary part.
3. The operator can scan or pick any of the listed parts (primary or substitute).
4. ION validates the scan against the primary and substitutes together, then accepts the install.
5. The aBOM line records what was installed, whether primary or substitute.

If the operator scans a part that's neither the primary nor an approved substitute, ION rejects the scan. From there, the path is one of these: configure the unexpected part as a substitute and retry, file an issue and document a deviation, or source the correct part.

## Substitutes vs revision interchangeability

Two related-but-different concepts:

| Concept                         | When to use                                                                                                               |
| ------------------------------- | ------------------------------------------------------------------------------------------------------------------------- |
| **Substitute**                  | Different part numbers that are functionally equivalent (for example, `FAST-M3-A` and `FAST-M3-B` from two suppliers)     |
| **Revision interchangeability** | Same part number, different revisions (for example, `BRKT-001 Rev A` and `BRKT-001 Rev B`), configured on the part itself |

ION treats these separately because they have different controls. Substitutes need explicit per-line approval. Revision interchangeability is set at the Parts Library level. For more information, see [Enable part interchangeability](/administration/supply-chain-settings/enable-part-interchangeability).

## mBOM substitutes and aBOM installations

A substitute is a plan that says "this is acceptable to install." The aBOM records what was installed. The two together let you answer questions such as:

* How often is the primary used compared to each substitute? Aggregate over installations.
* Does any unit contain the substitute you just recalled? Filter installations on `installedPartInventory.partId`.

You don't lose traceability when substitutes are used. The aBOM is precise about which part went in, even when the build requirement allowed several choices.

## Substitutes and reference designators

When a build requirement has both substitutes and reference designators, the operator picks a position and then scans the primary or any approved substitute for it. For the full positional rule, see [Reference designators](/build-hardware/bills-of-materials/reference-designators#substitutes-and-reference-designators).

<Tip>
  When you add a substitute, capture why it's equivalent in the build requirement notes, such as matching datasheet specs, finish, or plating. ION's analytics can surface how often each substitute is used compared to the primary.
</Tip>

## Related

* [Bills of materials overview](/build-hardware/bills-of-materials)
* [Manufacturing BOM (mBOM)](/build-hardware/bills-of-materials/manufacturing-bom)
* [As-built BOM (aBOM)](/build-hardware/bills-of-materials/as-built-bom)
* [Edit build requirements](/build-hardware/bills-of-materials/editing-build-requirements)
* [Reference designators](/build-hardware/bills-of-materials/reference-designators)