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.
Diskettes use one or two sectors per cluster for storage.
Cluster entries 0 and 1 are reserved by the operating system. Each cluster on the disk has a corresponding FAT entry. The FAT entry that corresponds to the first cluster of a file contains the number of the next cluster occupied by that file. By looking in the FAT entry corresponding to the next cluster, we find that either the end of the file has been reached or that the entry contains the number of yet another cluster occupied by the file. Thus the FAT entries effectively contain pointers to the clusters occupied by the file and to subsequent FAT entries that corresponds to additional clusters occupied by the file.