> ## 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.

# Reference designators

> Positional identifiers on a build requirement, including when to use them, how operators install against specific positions, and why they matter for traceability.

A reference designator pins a build requirement to a specific location in the assembly, such as `R7` or `U3`. See [Bills of materials overview](/build-hardware/bills-of-materials) for how designators sit on the line. When a build requirement has reference designators, operators install per designator rather than satisfying a count, and ION treats a requirement with no designators as quantity-only. This page covers configuring designators and installing against them.

## Configuring reference designators

On the build requirement's row in the **mBOM** card, with the version in **Draft**:

1. Open the **Reference Designators** dropdown in the row.
2. Type a designator and press **Enter** (or click **Add**). The input keeps focus so you can add them in sequence, for example `R1`, `R2`, through `R12`.
3. To remove one, click the trash icon next to it in the dropdown.

Keep the number of designators aligned with the quantity, one per position.

Each designator is unique within the build requirement. Two build requirements on the same assembly can both have an `R7`, but within one build requirement `R7` appears once.

## How operators install with reference designators

At run time:

1. The build requirement renders with one row per designator instead of a single quantity counter.
2. For each designator, the operator selects (or scans) the part inventory to install at that position.
3. ION records the (designator, installed part inventory) pair on the aBOM.
4. The build requirement is satisfied when every designator has an installation.

Operators can install in any order, because designators don't impose sequencing. If the assembly's procedure has a specific order, that's enforced at the procedure-step level.

## Substitutes and reference designators

Substitutes are configured at the build-requirement level, not per designator, so an approved substitute applies to every position the requirement covers. At install time, for each designator the operator can install the primary or any approved substitute, independently per position. So `R1` can take the primary while `R3` takes a substitute, and the aBOM records which part went into which position.

Because substitutes can't be scoped to individual designators, if `R3` and `R7` need different alternates, make them different build requirements.

## Designator naming conventions

ION doesn't enforce a designator format, so it's free text. Your downstream traceability is more useful if designators are consistent. Common patterns:

| Asset class                            | Convention                         |
| -------------------------------------- | ---------------------------------- |
| Resistors                              | `R1`, `R2`, …                      |
| Capacitors                             | `C1`, `C2`, …                      |
| Integrated circuits                    | `U1`, `U2`, …                      |
| Connectors                             | `J1`, `J2`, …                      |
| Transistors / FETs                     | `Q1`, `Q2`, …                      |
| Diodes                                 | `D1`, `D2`, …                      |
| Mechanical fasteners (when positional) | `S1`, `S2`, … or `M3-1`, `M3-2`, … |

If you use schematic or PCB designations from your design package, mirror them in ION. The alignment makes engineering and manufacturing handoffs easier.

<Tip>
  Include reference designators when there's a chance you'll need positional traceability. Adding them later requires reinstalling every affected unit.
</Tip>

<Tip>
  Match the engineering source of truth. If your PCB layout calls a position `U3`, the build requirement should call it `U3`. Renaming for clarity introduces drift.
</Tip>

<Note>
  Don't use reference designators for sequential count tracking. A torque pattern that needs fastener 1, then fastener 2, then fastener 3 is a procedure-step concern, not a designator.
</Note>

## Related

* [Bills of materials overview](/build-hardware/bills-of-materials)
* [Manufacturing BOM (mBOM)](/build-hardware/bills-of-materials/manufacturing-bom)
* [Edit build requirements](/build-hardware/bills-of-materials/editing-build-requirements)
* [Alternates and substitutes](/build-hardware/bills-of-materials/substitutes)