Why using a single index for a certain key field of a data record? Indexes can be obviously built for each field that uniquely identifies a record (or set of records within the file), and whose type is amenable to ordering. Multiple indexes hence provide a high degree of flexibility for accessing the data via search on various attributes; this organization also allows the use of variable length records (containing different fields).
It should be noted that when multiple indexes are are used the concept of sequentiality of the records within the file is useless: each attribute (field) used to construct an index typically imposes an ordering of its own. For this very reason is typicaly not possible to use the ``sparse'' (or ``spaced'') type of indexing previously described. Two types of indexes are usually found in the applications: the exhaustive type, which contains an entry for each record in the main file, in the order given by the indexed key, and the partial type, which contain an entry for all those records that contain the chosen key field (for variable records only).