@janstack Read and write index permissions are assigned with a role. You need read permissions to use field-level security as first you read documents from the index and then filter out the fields with FLS.
I don’t think there is permission that allows you to write to specific fields in the document.
@janstack You can write to specific fields in OpenSearch. Then, the rest of the fields in that single document will be empty. This behaviour is not related to the security plugin.
You can’t write twice to the same document using different fields as OpenSearch is the search tool and not the database.