Data contains the secret data. Each key must be a valid DNS_SUBDOMAIN or leading dot followed by valid DNS_SUBDOMAIN. The serialized form of the secret data is a base64 encoded string, representing the arbitrary (possibly non-string) data value here. Described in https://tools.ietf.org/html/rfc4648#section-4

stringData allows specifying non-binary secret data in string form. It is provided as a write-only convenience method. All keys and values are merged into the data field on write, overwriting any existing values. It is never output when reading from the API.