When processing a dimension in SSAS all of the members are assigned internal ids (dataid) which is sequentially assigned based on the order that the members are loaded.
This order is determined by the output of a select statement that is used to generate the distinct list of members and, if applicable, any related attributes.
This does not always result in a correctly ordered list of members which causes a problem when using partitions. SSAS assigns a minimum and maximum id for the members contained in each partition, and if the members are not ordered correctly, these ids will overlap and the query engine will have to scan more partitions than is necessary.
The order of the dataids should be the same as the order of the members when the cube is fully procesed. As defined by the OrderBy and OrderByAttribute properties of the Dimension Attribute in BIDs.