What is the Six character ID?

I found it just after I posted

it's used to identify a post (crafts, forums, planets, etc.) with a specific 6-digit code; for example, this post has 197159, and while this is purely numbers there are many that contain letters too
this code is only really used when redirecting old posts to a newer version of that post, otherwise it doesn't do much but still is needed

a joke?