The default member of an attribute hierarchy is used to evaluate expressions when an attribute hierarchy is not included in a query. The default member is ignored whenever a query includes an attribute hierarchy or user hierarchy that contains the attribute that sources the attribute hierarchy. This is because the member specified in the query is used.
The default member for an attribute hierarchy is set by specifying an attribute member as the DefaultMember property value for the attribute hierarchy. You can set this property on the Dimension Structure tab in Dimension Designer, or in the cube's calculation script on the Calculation tab in Cube Designer in SQL Server Data Tools (SSDT). You can also specify the DefaultMember property for a security role (overriding the default member set on the dimension) on the Dimension Data tab when defining dimension security. To avoid name resolution problems, define the default member in the cube's MDX script in the following situations: if the cube refers to a database dimension more than once, if the dimension in the cube has a different name than the dimension in the database, or if you want to have different default members in different cubes.
The default member on an attribute is used to evaluate expressions when an attribute is not included in a query. The default member for an attribute is specified by the DefaultMember property on the attribute. Whenever a hierarchy from a dimension is included in a query, all the default members from attributes corresponding to levels in the hierarchy are ignored. If no hierarchy of a dimension is included in a query, default members are used for all the attributes in the dimension.
Resolving the Default Member When No Default Member is Specified
If no default member is specified for an attribute hierarchy, and the attribute hierarchy is aggregatable (the IsAggregatable property on the attribute is set to True), the (All) member is the default member. If no default member is specified and the attribute hierarchy is not aggregatable (the IsAggregatable property on the attribute is set to False), a default member is selected from the attribute hierarchy's top level.
Specifying the Default Member
Every attribute in a dimension in Microsoft SQL Server Analysis Services has a default member, which you can specify by using the DefaultMember property for an attribute. This setting is used to evaluate expressions if an attribute is not included in a query. If a query specifies a hierarchy in a dimension, the default members for the attributes in the hierarchy are ignored. If a query does not specify a hierarchy in a dimension, the DefaultMember settings for dimension attributes take effect.
If the DefaultMember setting for an attribute is blank and its IsAggregatable property is set to True, the default member is the All member. If the IsAggregatable property is set to False, the default member is the first member of the first visible level.
The DefaultMember setting for an attribute applies to every hierarchy in which the attribute participates. You cannot use different settings for different hierarchies in a dimension. For example, if the  member is the default member for a [Year] attribute, this setting applies to every hierarchy in the dimension. The DefaultMember setting in this case cannot be  in one hierarchy and  in a different hierarchy.
If you define a default member for a particular level in a hierarchy that does not aggregate naturally, you must define default members in all levels above that level in the hierarchy. For example, in the hierarchy All-Countries–Climate, you cannot define a default member for Climate unless you define a default member for Countries. Failing to do so creates query-time errors.
When levels in a hierarchy aggregate naturally, you can define a default member for any attribute in the hierarchy without regard to other attributes in the hierarchy. For example, in the hierarchy Country–Province–City, you can define a default member for City such as [City].[Montreal] without defining the default member for State or for Country.