An argument is an information entity. Search terms, expressions, operators and its operands, and arguments are of a certain type. The type of operands must fit the type expected by the operator. Operators are further discussed in Section 6.2.3 . This makes it possible not only to search for certain language constructs, but use object information as well.
Objects hold a lot of information encoded in their representations.
Each media type allows searching to some extent. And each media type has different strengths and weaknesses in regard to searching. As already discussed in Section 6.2.1 the concept of an object ties these alternative representations together. Thus it is valid to combine searches of different media types to create one search term. This takes advantage of the strengths, in terms of searchability, of each media type.