definition of filtering, polyinstantiation

The apparent key of a multilevel relation is the set of attributes that would have
formed the primary key in a regular (single-level) relation. A multilevel relation will
appear to contain different data to subjects (users) with different clearance levels.


In some cases, it is possible to store a single tuple in the relation at a higher classification
level and produce the corresponding tuples at a lower-level classification
through a process known as filtering.


In other cases, it is necessary to store two or more tuples at different classification levels with the same value for the apparent key.
This leads to the concept of polyinstantiation, where several tuples can have the
same apparent key value but have different attribute values for users at different
clearance levels.