yes, i understand-- but while i might come across a duty roster at work-- i don't think roster is much used in dorms, or other locations.. so rota is extremely obscure, and roster is a word with a very limited use (in my experience)-- I have no trouble understanding the meaning, (and gathered that it was a not uncommon word for UK english users-) its just roster is not used very much in US english.. We have schedules of events, and schedules for car pools, and schedules for who is on "duty" in the dorm. it might even be labeled a duty roster-- but we would speak of "who's scheduled? -- check the schedule..." and not "check the duty roster..."