# Conditional Display and Validation

Any custom field can be gated on the value of another field in the same intake form by adding a `display_condition` rule. The Privacy Center evaluates the rule against the values the data subject has entered so far and only renders the dependent field when the rule resolves to `true`.

`display_condition` accepts either a **single rule** that compares one field, or a **combined rule** that joins multiple rules together with a logical operator.

```json
"reason_details": {
  "label": "Tell us more",
  "field_type": "textarea",
  "required": false,
  "display_condition": {
    "field_address": "reasons",
    "operator": "list_contains",
    "value": "Other"
  }
}
```

A combined rule joins multiple single rules with `and` or `or`:

```json
"california_resident_proof": {
  "label": "Proof of California residency",
  "field_type": "file",
  "required": true,
  "display_condition": {
    "logical_operator": "and",
    "conditions": [
      { "field_address": "state", "operator": "eq", "value": "California" },
      { "field_address": "marketing_opt_out", "operator": "eq", "value": true }
    ]
  }
}
```

## Rule shape

A **single rule** has three attributes:

| Attribute | Description |
| --- | --- |
| `field_address` | Key of another field declared in the same `custom_privacy_request_fields` object. Referencing an unknown key is rejected. |
| `operator` | One of the supported operators below. |
| `value` | Value compared against the referenced field. Required for every operator except `exists` and `not_exists`, which must be supplied without a `value`. |

A **combined rule** has two attributes:

| Attribute | Description |
| --- | --- |
| `logical_operator` | Either `and` or `or`. |
| `conditions` | A non-empty list of single rules or nested combined rules. |

## Supported operators

The set of operators allowed inside `display_condition` is a deliberate subset of the broader Fides condition operators:

| Operator | Meaning | Requires a `value`? |
| --- | --- | --- |
| `eq` | Referenced field equals `value`. | Yes |
| `neq` | Referenced field does not equal `value`. | Yes |
| `exists` | Referenced field has been filled in (non-null). | No |
| `not_exists` | Referenced field has not been filled in. | No |
| `list_contains` | The referenced multi-value field contains `value`, or `value` (when a list) intersects the referenced field. | Yes |

## Operator and value compatibility

The operator and the shape of `value` must be compatible with the `field_type` of the referenced field. The matrix below summarizes the permitted combinations; any other combination is rejected when the Privacy Center configuration is saved.

| Referenced `field_type` | Allowed operators | `value` shape |
| --- | --- | --- |
| `text`, `textarea`, `select`, `location` | `eq`, `neq`, `exists`, `not_exists` | String |
| `checkbox` | `eq`, `neq`, `exists` | Boolean (`true` / `false`) |
| `checkbox_group`, `multiselect` | `list_contains`, `exists`, `not_exists` | String, or a list of strings |
| `file` | `exists`, `not_exists` | — |

`display_condition` rules are validated at configuration save time. Misconfigured rules — unknown `field_address`, an operator that is not allowed for the referenced `field_type`, a `value` whose type does not match the target, or a `value` supplied with `exists`/`not_exists` — are rejected with `422 Unprocessable Entity` so that broken rules cannot reach the rendered intake form.