type of data: text strings
data structure: php object
when it gets stored: on load of a preference
where it gets stored: seems to be just a property of a user - often $USER
how it gets read: using get_user_prefence reads from $user->preference array of items
does it need locking:
how it gets cleared: using cache_flags (get_cache_flag), ttl '_lastloaded'
typical sizes:
safeguards in place (eg limits): get_cache_flag to update