Occupation Ceilings refer to the limit on the number of invitations that can be issued through SkillSelect each year per occupation group. This ensures that the skilled migration program is not dominated by a small number of occupations. Once this limit is reached, no further invitations for that particular occupation group will be issued for that program year. Invitations will then be allocated to intending migrants in other occupation groups even if they are lower ranking.
*Source : https://immi.homeaffairs.gov.au/visas/working-in-australia/skillselect/occupation-ceilings