If O1, O2, ...., On are objects, so is f({O1, O2, ..., On}), where f is an operator from a set of objects to an object. In general, an object O is a well-formed expression involving other objects Oi, an operator f, the opening bracket '{' and the closing bracket '}'. We write O = f({O1, O2, ..., On}). The objects O, O1, O2, ..., On are related objects.

If Oi is related to Oj and Oj is related to Ok, then Oi is related to Ok. A document is a collection of related objects.

Each object Oi has a profile Pi = (Pi1, Pi2, ...., Pim) where each attribute Pij denotes some characteristics of Oi.