MDAC 2.5 SDK - OLE DB Programmer's Reference
Chapter 25: MDX Grammar

Leveling Rules for MDX Search Condition

If <value_expression>s in a <boolean_primary> value are <string_value_expression>s, support for <comp_op> values other than <equals_operator> and <not_equals_operator> is optional. Consumers can check the value of the property MDPROP_MDX_STRING_COMPOP to see whether a provider supports this feature.