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---
# Make a field mandatory with a rule

**Goal:** a field must be filled — but only when another field's value says so. Working example: *FATCA Declaration is mandatory when Customer Type = NRI; residents can leave it blank.*

This is a [Mandatory rule](https://knowledge.leegality.com/document-execution/workflows/v4/rule-engine/rule-categories#a--mandatory-rules), and it ships the way mandatory rules typically do: as a **validation at Proceed** — the rule checks the field when the user clicks Proceed and blocks the stage with an error naming exactly what's missing.

## The spec

```
TARGET field:   FATCA Declaration (pack-level custom field)
ACTION:         Set Mandatory = true
SUBJECT field:  Customer Type (pack-level custom field, Dropdown: NRI / Resident)
CONDITION:      Customer Type = NRI
OUTCOME:        NRI runs cannot Proceed with FATCA empty (error shown);
                Resident runs proceed freely with FATCA blank
Workflow:       (yours)
```

## The rule logic

This assumes both custom fields are added at **pack level**, with API names `customerType` and `fatcaDeclaration`:

```
#version 2.0
var customerType = pack.lecf_customerType; if(customerType != null && (isEmpty(customerType) == false)) { if(customerType == "NRI") { var fatca = pack.lecf_fatcaDeclaration; if(fatca == null || isEmpty(fatca)) { throw "FATCA Declaration is mandatory for NRI customers"; } } }
```

Reading it against the spec: the rule reads the Subject Field, checks the Condition (`== "NRI"`), and enforces the Target by throwing the error only when the field is empty **and** the condition holds. Below/outside the condition, the rule does nothing — that's the OUTCOME's other side.

## Adapt these parts

| Part | In the logic | Change it to |
| --- | --- | --- |
| Subject Field | `pack.lecf_customerType` | `lecf_` + your field's exact API name |
| Condition value | `"NRI"` | The exact dropdown option, spelled as configured |
| Target Field | `pack.lecf_fatcaDeclaration` | `lecf_` + the field being made mandatory |
| Error message | `"FATCA Declaration is mandatory for NRI customers"` | Your message, word for word — it's part of the spec |

For a **numeric** condition instead of a dropdown match (mandatory above a threshold), the comparison needs a number conversion — that variant is the [Build your first rule](https://knowledge.leegality.com/document-execution/workflows/v4/rule-engine/build-your-first-rule) tutorial, end to end. If your fields live at **document or invitee level** rather than pack level, the paths differ — send the spec to [support@leegality.com](mailto:support@leegality.com) or see Advanced: LEEQL for the data model.

## Configure it

1. Rule Engine panel → new rule → **Rule Title**: `FATCA mandatory for NRI customers`.
2. Paste the adapted logic into **Create Rule**.
3. **Trigger Method**: **On Proceed Create** — pack-level fields are completed in the Create stage, so the check belongs on that stage's Proceed. (If your target field is filled on the Invite stage, use **On Proceed Invitee** instead — pick the Proceed of the stage where the field gets filled.)
4. Save. Full configuration flow, step by step: [Build your first rule](https://knowledge.leegality.com/document-execution/workflows/v4/rule-engine/build-your-first-rule).

## Test it — both directions

1. Customer Type = NRI, FATCA empty, Proceed → blocked with your error.
2. Fill FATCA, Proceed → goes through.
3. Customer Type = Resident, FATCA empty, Proceed → goes through, no error.

## Related

Mandatory also works in reverse — relaxing a requirement when a condition holds — see [Rule categories](https://knowledge.leegality.com/document-execution/workflows/v4/rule-engine/rule-categories#a--mandatory-rules).