Private list for tracking. (There was a list of accounts, available in the history.)
Usually replace entire page with some "FU** YOU!!!!!"(1, 2, 3 ("Why are you blocking me you son of a bitch"), ...) or personal attack (sometimes libel) about admins who blocked his previous sock ("Unblock me $blocking_admin you such disabled bitch"). When blocked, he edits his user talk page to continue vandalising, so disabling talk page access is required.
CUing for sleeper is always futile. They don't stack accounts for future vandal'ing. Suspected as spawning accounts from mobile ISP.