One particular instance a general rule must meet. Also the container a thing is kept in.
Two separate Latin words fell together in English spelling — casus (a fall, an occurrence, from cadere, to fall) behind the 'instance of a rule' sense, and capsa (a box, from capere, to hold) behind the 'container' sense — so one English word now carries both histories at once.
One particular instance a general rule must answer to, and also the container a thing is kept in — two old words wearing one spelling.