By default, the rusage string reserves resources for running jobs. Because resources are not reserved for pending jobs, some memory-intensive jobs could be pending indefinitely because smaller jobs take the resources immediately before the larger jobs can start running. The more memory a job requires, the worse the problem is.
Memory reservation for pending jobs solves this problem by reserving memory as it becomes available, until the total required memory specified on the rusage string is accumulated and the job can start. Use memory reservation for pending jobs if memory-intensive jobs often compete for memory with smaller jobs in your cluster.
Unlike slot reservation, which only applies to parallel jobs, memory reservation applies to both sequential and parallel jobs.