# Parts and revisions

> Part kinds, the revision lifecycle, released means immutable.

A **part** is anything you build, buy or stock. A **revision** is one version of a part's design. This page explains both, because every other part of BuildTrace hangs off them.

You work with parts and revisions in **Parts & BOMs**. Create a part with `createPart`. Create a revision with `createRevision`.

## Part kinds

Every part has one of three kinds. The kind decides how BuildTrace tracks stock of the part.

| Kind | What it is | How it is tracked |
|---|---|---|
| `RAW` | Bulk material such as resin, wire or screws | By quantity, per location and lot |
| `COMPONENT` | A bought-in or made item you count one by one | By serial number |
| `ASSEMBLY` | Something you build from other parts | By serial number |

`COMPONENT` and `ASSEMBLY` parts are serialized: each unit has its own serial number and its own history. `RAW` parts are tracked by quantity only. See [Inventory](https://dev.buildtrace.eu/docs/concepts/inventory.md) and [Serials](https://dev.buildtrace.eu/docs/concepts/serials.md).

You can change a part's kind later with `reclassifyPart`.

## Revisions

A part can have many revisions. Each one has a label that you choose, such as `A`, `B` or `v2`. A part cannot have two revisions with the same label.

A revision has one of two statuses:

- `DRAFT`: you can still change its BOM and process.
- `RELEASED`: it is final.

Open the **Revisions** tab to see every revision of the selected part, its status and its release date. Select **New revision** to create a draft. You can copy the BOM from an earlier revision when you do.

## Released means immutable

Selecting **Release** is permanent. A `RELEASED` revision is `immutable`: you can't edit its BOM lines, and you can't change its work instructions or test plan. The database blocks the edit, not only the screen.

Release a revision with `releaseRevision`. Two rules apply:

- Every child revision that the BOM pins must already be released. If not, the release fails and lists the ones to release first.
- Only a `DRAFT` revision can be released.

## Changing a released revision

Because a released revision can't change, you have two ways to move forward:

- Create a new revision. Copy the old one and edit the copy.
- Raise a change order against the released revision. See [Change orders](https://dev.buildtrace.eu/docs/concepts/change-orders.md).

Re-baselining combines both. It folds the change orders that are in force into a new draft revision. See [Change orders](https://dev.buildtrace.eu/docs/concepts/change-orders.md).

## Compare two revisions

On the **Revisions** tab, choose two revisions and select **Compare**. BuildTrace lists the parts that were added, removed or changed between them. See [Bills of materials](https://dev.buildtrace.eu/docs/concepts/boms.md).