#SESSION{parameter} displays data about a connected visitor. A session may be considered as a set of some data, stored on the server whilst the visitor is still connected. As such, this data can be retrieved and re-used when the visitor changes pages by navigating through the site.
The existence of this tag, as with the #AUTORISER tag, generates a different cache for each authenticated visitor on the site, and one more additional cache for the non-authenticated visitors.