anchor.diff:    "full_name": "Service account - elastic/fleet-server",
     "email": null,
     "token": {
  -    "name": "token1"      <1>
  +    "name": "token1",                 <1>
  +    "type": "_service_account_index"  <2>
     },
     "metadata": {
       "_elastic_service_account": true
     },
     "enabled": true,
     "authentication_realm": {
  -    "name": "service_account",
  -    "type": "service_account"
  +    "name": "_service_account",
  +    "type": "_service_account"
     },
     "lookup_realm": {
  -    "name": "service_account",
  -    "type": "service_account"
  +    "name": "_service_account",
  +    "type": "_service_account"
     },
     "authentication_type": "token"
   }
   ----
   // NOTCONSOLE
  -<1> Name of the service account token
  +<1> Name of the service account token.
  +<2> Type of service account token. The value always begins with
  +`_service_account_` and is followed by a string that indicates the service
  +token backend in use (can be either `file` or `index`).
  
positive.code: // NOTCONSOLE
  
  A successful authentication response includes a `token` field, which contains a
  `name` for the name of the service account token:
  
  [source,js]
  ----
  
negative.code: 
  The {stack-security-features} provide _service accounts_ specifically for
  integration with external services that connect to {es}, such as {fleet} server.
  Service accounts have a fixed set of privileges and cannot authenticate
  until you create a service account token for them. Additionally, service
  accounts are predefined in code, and are always enabled.
  
  A service account corresponds to a specific external service. You create service
  account tokens for a service account. The service can then authenticate with the
  token and perform relevant actions. For example, {fleet} server can use its
  service token to authenticate with {es} and then manage its own API keys.
  
  You can create multiple service tokens for the same service account, which
  prevents credential sharing between multiple instances of the same
  external service. Each instance can assume the same identity while using
  their own distinct service token for authentication.
  
  Service accounts provide flexibility over <<built-in-users,built-in users>> 
  because they:
  
  * Do not rely on the <<native-realm,internal `native` realm>>, and aren't
  always required to rely on the `.security` index
  * Use a role descriptor named after the service account principal instead of 
  traditional roles
  * Support multiple credentials through service account tokens
  
  Service accounts are not included in the response of the
  <<security-api-get-user,get users API>>. To retrieve a service account, use the
  <<security-api-get-service-accounts,get service accounts API>>.
  
  [discrete]
  [[service-accounts-explanation]]
  === Service accounts usage
  Service accounts have a
  <<security-api-get-service-accounts-path-params,unique principal>> that takes
  the format of `<namespace>/<service>`, where the `namespace` is a top-level
  grouping of service accounts, and `service` is the name of the service and
  must be unique within its namespace.
  
  Service accounts are predefined in code. Currently, only one service account is available:
  
  `elastic/fleet-server`:: The service account used by the {fleet} server to
  communicate with {es}.
  
  // tag::service-accounts-usage[]
  IMPORTANT: Do not attempt to use service accounts for authenticating individual
  users. Service accounts can only be authenticated with service tokens, which are
  not applicable to regular users.
  // end::service-accounts-usage[]
  
  // tag::service-accounts-tls[]
  