> ## Documentation Index
> Fetch the complete documentation index at: https://developers.openalex.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Subfields Overview

> Schema reference for Subfields entities

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](/guides/filtering), [Sorting](/guides/sort), and [Grouping](/guides/grouping) for syntax.

Fields marked **(deprecated)** are `.search` filters — use the [`search` parameter](/guides/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`                          |   ✓  |     ✓     |    ✓   |