Update index.md

docs/en/04-concept/index.md

@@ -168,8 +168,15 @@ In a database, there can be one or more STables, but a STable belongs to only on
 
 ## Instance, URL, Token
 
-An instance is a running cluster of nodes of TDengine with one or more databases. An instance cannot span across multiple regions or multiple clouds, but a single account (organization) can have multiple instances. An account may invite multiple users into his/her organization to share the data, and each user can be configured with different access rights.
-
+An instance is a running cluster of nodes of TDengine with one or more databases. An instance cannot span across multiple regions or multiple clouds.
 TDengine cloud provides a unique URL for each instance and uses tokens to authenticate the access. The token is generated by TDengine cloud for each user and for each instance. The token has a duration and can be reset by the user for each instance at any time for security purpose.
 
+## User, Organization
+
+When a user register TDengine cloud service, an organization with one instance is created automatically. The user can add more instances in an organization later. In addition, the user can invite others to share data within his/her organization. He/she can share the whole organization, instance, a database or topics with fine tuned privileges to make sure data is securely shared.
+
+A user is uniquely identified by email address. He/she may belongs to multiple organizations. In different organization, he/she may have different roles and different access privileges.
+
+The billing is based on the usage for all the instances owned by an organization. 
+