The client will authenticate on the other system and get the sessionid.
Then it will authenticate on Lightstreamer Server by passing any of the credentials (username, epassword, sessionid) to the client API. Such credentials will be passed yo your Metadata Adapter, which will validate them.
Then the client will subscribe to an item, which should map your status change events. Such subscription will reach the Data Adapter, which will need to start producing events. The same subscription will go through the Metadata Adapter first, which will be able to decorate the item name with the sessionid or other information, so that the Data Adapter will know what data to produce.

To understand the above better, please study Chapter 4 of "General Concepts.pdf".