A job slot is the basic unit of processor allocation in LSF. A sequential job uses one job slot. A parallel job that has N components (tasks) uses N job slots, which can span multiple hosts.
By default, running and suspended jobs count against the job slot limits for queues, users, hosts, and processors that they are associated with.
With processor reservation, job slots that are reserved by pending jobs also count against all job slot limits.
When backfilling occurs, the job slots used by backfill jobs count against the job slot limits for the queues and users, but not hosts or processors. This means when a pending job and a running job occupy the same physical job slot on a host, both jobs count towards the queue limit, but only the pending job counts towards host limit.