A partition is based either on a table or view in a data source, or on a table or named query in a data source view. The location where partition data is stored is defined by the data source binding. Typically, you can partition a measure group horizontally or vertically:
In a horizontally partitioned measure group, each partition in a measure group is based on a separate table. This kind of partitioning is appropriate when data is separated into multiple tables. For example, some relational databases have a separate table for each month's data.
In a vertically partitioned measure group, a measure group is based on a single table, and each partition is based on a source system query that filters the data for the partition. For example, if a single table contains several months data, the measure group could still be partitioned by month by applying a Transact-SQL WHERE clause that returns a separate month's data for each partition.
It seems like it should be the opposite.