config.yaml

The heart of a Content Block is the config.yaml file. This YAML file defines both the available fields and the structure:

EXT:some_extension/ContentBlocks/ContentElements/content-block-name/config.yaml

name: vendor/content-block-name
fields:
  - identifier: header
    useExistingField: true
  - identifier: my_text_field
    type: Text
    max: 10

First of all, a name has to be defined. It must be unique inside your installation. It consists, similarly to composer package names, of a vendor and a package part separated by a slash. It is used to prefix new field names, new tables and record type identifiers.

Inside fields you define the structure and configuration of the necessary fields. The identifier has to be unique per Content Block.

It is possible to reuse existing fields with the flag useExistingField. This allows e.g. to use the same field header or bodytext across multiple Content Blocks with different configuration. Be aware that system fields shouldn't be reused. A list of sane reusable fields can be referenced in the documentation. Furthermore, own custom fields can be reused as well.

Refer to the YAML reference for a complete overview.
Learn more about reusing fields
Learn how to extend TCA of Content Blocks (for advanced users).
For more information about the YAML syntax refer to YAML RFC