apply_filters( 'relevanssi_index_attachment_revision_types', array $post_types )
When Relevanssi indexes posts, it also looks at the child posts. However, if the indexed post is a revision or an attachment, the child posts are not checked. You may extend this behaviour to other post types with this filter hook.
(array) An array of post type names, default
array( 'attachment', 'revision' ).
This filter hook was added to facilitate reading attachment content for other post types than
attachment, but actually this is not necessary for that purpose.
This filter hook can be used to make Relevanssi skip child post checks for post types other than
revision, but I’m not sure what purpose that would serve.
This filter hook may end up being removed at some point. If you have come up with some actual use for this filter hook, please let me know.