AUO returns one canonical entity per resolve, joined across every source. To use it well, it helps to understand three things about how Australian entities actually work.

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entity_form is a first-class discriminator

Every resolved entity has an entity_form, one of:
entity_formMeaningHas an ACN?
companyAn ASIC-registered company acting in its own rightYes
trustA trust (its own ABN; the trustee is a separate entity)No
sole_traderAn individual carrying on businessNo
partnershipA partnershipNo
indigenous_corporationA CATSI-Act corporation (ORIC)Sometimes
otherAny other incorporated or registered entityVaries
entity_form is derived from the Australian Business Register entity type and, for Indigenous corporations, corrected against the ORIC register. It tells you which enrichment blocks are meaningful for the entity.

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An ACN is not a sibling of an ABN

The common shortcut “an ABN is an ACN plus a two-digit prefix” holds only for a company acting in its own right. It breaks the moment a trust or a sole trader is involved.
A trust has an ABN but no ACN. Its corporate trustee is a separate entity with its own ABN and ACN. AUO never derives an ACN from a non-company ABN. When acn is null, the field carries a note explaining why.
The acn field on a non-company entity
"acn": {
  "value": null,
  "source": "ABR",
  "note": "trusts/sole traders/partnerships have no ACN; companies only"
}

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Trusts resolve to their corporate trustee

When you resolve a trust, the entity name often encodes its trustee, for example ACME PTY LTD AS TRUSTEE FOR THE SMITH FAMILY TRUST. AUO parses this into a related_entities edge of role corporate_trustee and makes a best-effort lookup of the trustee’s ACN in the ASIC company register. This matters for risk: liability sits with the trustee, not the trust. So when AUO screens a trust, it screens the resolved trustee, not the trust itself.
A resolved trustee edge
"related_entities": [
  {
    "role": "corporate_trustee",
    "name": "ACME PTY LTD",
    "abn": "12345678901",
    "acn": "345678901",
    "resolvable": true
  }
]
If the trustee cannot be resolved on free sources, the edge is still returned with resolvable: false and trustee_resolution: "unavailable_on_free_sources", so you always know what AUO could and could not confirm.

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Everything is one canonical entity

A single /resolve response joins the Australian Business Register spine with, where applicable:

Company data

ASIC company status, current and former names, registration dates.

Charity

ACNC registration, charity size, registration date.

Licensing

AFS and credit licensees and representatives.

Professional registrations

Financial advisers, registered auditors, SMSF auditors.

External IDs

The GLEIF LEI cross-walk (opencorporates, qcc, spglobal).

Screening

Sanctions, banned persons and organisations, insolvency, external administration.
Each block is populated only when the entity matches that source, and each carries its own provenance. Blocks that do not apply to an entity type return a safe empty shape rather than being omitted, so your integration can rely on a stable schema.

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Provenance

How every field carries its source and as-of date.

Honest boundaries

What AUO does and does not cover, stated plainly.