Enumerates criterion system serving statuses.

[Proto definition ![GitHub](https://developers.google.com/site-assets/logo-github.svg)](https://github.com/googleapis/googleapis/blob/master/google/ads/googleads/v25/enums/criterion_system_serving_status.proto)

| Enums ||
|---|---|
| ## `ELIGIBLE` | Eligible. |
| ## `RARELY_SERVED` | Low search volume. |
| ## `UNKNOWN` | The value is unknown in this version. |
| ## `UNSPECIFIED` | Not specified. |