All search expressions (filters) in Exchange Web Services are exposed as descendants of the SearchExpressionType schema type. SearchExpressionType is an abstract type, meaning that you will never encounter one in an instance document. Figure 14-1 shows the search expression class hierarchy exposed by Exchange Web Services.
Figure 14-1. SearchExpression Hierarchy
A restriction has a tree structure. The Restriction element is always the root node of this tree. From there, the tree expands out based on the various filters contained within the restriction. Following the nature of a tree, you have branches and you have leaves. A branch ...