This morning I've posted this question to stackoverflow forum but it seems it got unnoticed. Probably I should have posted it here in the first place.
I'm trying to get some understanding of how WordPress works and while inspecting the database I found that, in a multisite installation, there is a table called wp_site that seems always holding a single record with a corresponding site_id. I cannot understand (nor find on internet) what is the use of site_id when actually multisites seem to use wp_blogs and corresponding blog_id for identifying different sites.(http://codex.wordpress.org/Database_Description)
Can someone shed some light on what should be my understanding of site_id?