# How can I track uploads for a specific user?

Use Assembly fields to attach correlation metadata, such as a user ID or your application’s upload reference. With [Uppy’s Transloadit plugin](https://uppy.io/docs/transloadit/#assemblyoptions), use `assemblyOptions` to obtain `params`, `signature` and optional `fields` from your backend.

For example, `assemblyOptions.fields: { user_id: 'user-123' }` makes the value available as `${fields.user_id}` in instructions and in the `fields` **object** in Assembly Status. See [fields in Templates](/docs/topics/templates.md#overruling-templates-at-runtime).

Uppy sends these fields as separate multipart parts, outside the serialized `params` string. They are client-controlled. Even a field inside signed `params.fields` can be replaced in the merged fields by a later multipart field with the same name. Do not use returned fields alone to authenticate a user, choose an authorized storage destination or grant access to results.

For an association controlled by your application, follow [verifying an upload’s user](/docs/faq/validate-users.md).