# Write Code for a Use Case

## Intent

A use case is one command or query with its access, results, and scenarios, and its code spreads across every backend layer. This page maps each code element of a use case to its folder, its mark, and its check, so an author places new code without hunting six pages. The [marks table](https://litespec.dev/4.0.0/core/model.md#marks) owns which mark each element carries; the stack pages state each mark's written form.

## Concepts

### The Element Table

Each row names one code element, the folder that holds it, the mark it carries, and the check that watches it. A check cell naming codes names the findings the tools report; a check cell naming review names the code a reviewer reads. This page adds no rule of its own.

| Element | Goes In | Mark | Checked By |
|:---|:---|:---|:---|
| Command or query handler | The use case folder | `implements` naming `use-case/<module>.<name>` | `use-case-act-missing`, `write-undeclared`, `effect-undeclared` |
| Command or query message | The use case folder beside its handler | None; the handler folder places it | `name-drift` |
| Validator | The use case folder beside its message | `implements` naming `use-case/<module>.<name>` | Review the validator body |
| Result type | The use case folder beside its handler | None; the handler folder places it | `result-undeclared` |
| Aggregate root | The aggregate `Domain/` folder | `implements` naming `aggregate/<module>.<name>` | `aggregate-unimplemented` |
| Root method that changes state | On the aggregate root | `implements` naming `use-case/<module>.<name>`, exactly one | `use-case-act-missing`, `mark-count-invalid`, `act-event-undeclared`, `act-event-unimplemented` |
| Domain event | The `Events/` folder of its aggregate `Domain/` folder | `implements` naming `event/<aggregate>.<fact>` | `act-event-undeclared`, `act-event-unimplemented`, `code-placement-drift` |
| Refusal type | The `Refusals/` folder of its aggregate `Domain/` folder | `implements` naming `rule/<home>.<name>` | `refusal-unimplemented` |
| Repository interface | The aggregate `Domain/` folder | None; the aggregate folder places it | `constituent-parent-missing` |
| Repository implementation | The aggregate `Infrastructure/` folder | `implements` naming `aggregate/<module>.<name>`, or derived from the interface | `repository-unimplemented` |
| Endpoint | The use case folder, or the host for a host-only endpoint | `implements` naming `use-case/<module>.<name>` | `use-case-endpoint-missing`, `operation-use-case-drift` |
| Request and response models | The use case folder beside the endpoint, or the node `WebApi/` folder when shared | None; the endpoint folder places them | Review the transport models |
| Event handler class | The `UseCases/On{Fact}` folder inside the module `UseCases/` folder | `implements` naming `reaction/<module>.<name>` | `reaction-unimplemented`, `effect-undeclared` |
| Scheduled or manual host | The module `Jobs/` folder, hosted in Infrastructure or Worker | `implements` naming `job/<module>.<name>` | `job-unimplemented` |
| External-system port | The node `Application/` folder beside the aggregate it serves, or the owning module `Contracts/Ports/` folder when called from another module | `implements` naming `external-system/<name>` | `external-system-unimplemented` |
| Test | The test suite for its boundary, mirroring the module tree | `proves` naming `scenario/<use-case>.<name>` | `proves-unreached`, `use-case-scenario-missing`, `use-case-unverified` |

Screens stay outside this table: whether a browser reaches a use case depends on the deployment. The [Project Model](https://litespec.dev/4.0.0/core/model.md) states the mark rules, and [System](https://litespec.dev/4.0.0/core/system.md) requires each mark; each stack page states the written form its language takes.

## Standards

None.

## Conventions

None.