The FAT is capable of files and parts of files in a random manner. This means that files do not have to be stored in contiguous sectors. If the space is not large enough to accommodate the new file, sufficient disk space must be allocated at the end of the file storage. If the disk does not contain sufficient space for the file at the end, the disk is considered full. This can cause a significant waste of storage space, especially on the smaller capacity diskettes.
Random storage enables a file to be stored in multiple areas on the disk.
The FAT is the mechanism for keeping track of files multiple locations. Files are referenced in the FAT in terms of clusters. A file always occupies at least one cluster, even if it is only a one-byte file. If the file is large enough, it will occupy multiple clusters. A cluster is really an allocation unit.