Occupation Ceilings refer to the limit on the number of invitations that can be issued through SkillSelect each year.
Each occupation group has an occupation ceiling associated. Once an occupation group has reached its ceiling, invitations cannot be issued for that programme year. This means that applicants will not be invited to apply for specified visas regardless of their points.
Source: Australian Government Department of Home Affairs | SkillSelect | Occupation ceilings - https://www.homeaffairs.gov.au/Trav/Work/Skil