What is the meaning of NCA vacancies seen in most job applications?

NCA means No CAndidate Available

Sometimes the required number of candidates to fill up the vacancies reserved for them may not be available in the Ranked list.This situation is termed as NCA.In such cases the following procedure is being followed.If a suitable candidate is not available for selection from any particular community or group of communities such vacancy shall be kept unfilled, notified separately for that community or group of communities for that selection year and shall be filled by direct recruitment exclusively from among that community or group of communities