Item

Returns one member from set with specified position (starting from zero) or name.

Syntax

Index syntax
Set_Expression.Item(index)

String expression syntax
Set_Expression.Item(String_Expression1 [ ,String_Expression2,...n])

Set_Expression	MDX expression that returns set.
index	Numeric expression that specifies the position of tuple in the set.
String_Expression	String expression that is a typically a tuple expressed in a string.

Following example returns all Projects that do not have component "UI".

Aggregate(Filter([Project].[Project].Members,
  [Project].CurrentMember.Children.Item('UI') IS NULL
))

[Project].CurrentMember.Children returns all project components that are Project dimension Children.
.Item('UI') returns component with name "UI"
If component with specified name does not exist, NULL will be returned. In that way IS NULL filters projects that do not have component "UI".