# Subfields Overview

Subfields are the third level of the OpenAlex topic hierarchy. There are 254 subfields spread across 26 fields. Each subfield contains multiple topics.

The topic hierarchy is: Domain → Field → **Subfield** → Topic

## Filter, sort, and group_by fields

The API supports filtering, sorting, and grouping on subfields fields. See [Filtering](/api/filtering/), [Sorting](/api/sorting/), and [Grouping](/api/grouping/) for syntax.

Fields marked **(deprecated)** are `.search` filters — use the [`search` parameter](/api/searching/) instead.

| Field | Sort | Group_by | Filter |
|-------|:----:|:--------:|:------:|
| `cited_by_count` | ✓ | ✓ | ✓ |
| `default.search` **(deprecated)** |  |  | ✓ |
| `display_name` | ✓ |  | ✓ |
| `display_name.search` **(deprecated)** |  |  | ✓ |
| `domain.id` | ✓ | ✓ | ✓ |
| `field.id` | ✓ | ✓ | ✓ |
| `from_created_date` | ✓ |  | ✓ |
| `id` | ✓ | ✓ | ✓ |
| `topics.id` | ✓ | ✓ | ✓ |
| `works_count` | ✓ | ✓ | ✓ |