The main source of disk space for each user is in their home directory.
Home directories are on disks provided by the department for general use.
Home disks are shared by dozens, or even hundreds of users. When a home disk fills up, no user on that disk can work. As a result, it is very important to keep strict control on the amount of disk space allocated to each user. This is done using disk quotas. A disk quota involves 2 numbers, a soft and a hard quota.