Well as mentioned above they do not get allocated: so they can either sit in css exams next year, or they can await result of the other attempt if they appeared while they waited for original attempt
i.e. A person who cleared written in css 2018 but does not get allocation in upcoming “final” result of css 2018, but appeared in css 2019 as he/she was waiting for css 2018’s result can either wait for css 2019 result, and/or also start prep for css 2020 if they have an attempt left