The GlideSession class has no constructor, but it is returned from the getSession() method of the GlideSystem API (gs.getSession();). This API allows you to get and set data related to the current session.
The getClientData() and putClientData() methods allow you to set data in the user's session from a server-side script using putClientData(), and then retrieve that data using getClientData().
This functionality can be extremely useful for passing data back-and-forth between the server and client, without performance-costly synchronous queries or time-consuming Ajax calls.
In business rule on the Incident table, we can place some data in the user's client session like so:
var userSession = gs.getSession();
session.putClientData('ticket_number', current.getValue('number'));
And then in a client script, you can access this data like so:
var ticketNumber = g_user.getClientData('ticket_number');