The UOSFS is a complex data structure which exists on a store.
Disks are one of the type of stores that UOS uses. Disks can read and write entire sectors, the minimum writable number of bytes is called the cluster size.
The File System will need to allocate chunks of the available storage space as required. Also, as things are deleted, it will need to deallocate those chunks so they can be reused for other purposes. The management of free and allocated space is the domain of the store itself - not the file system. A managed store keeps track of free and allocated space, which is typically done through an allocation table (a data structure that allows the store to manage the space). The UOS File System can deal with any cluster size of 256, or more, bytes.
The allocation table is a bit array, where each bit represents a cluster on the store. If the bit is set, the corresponding cluster is in use; if it is not set, the cluster is available.