The Usage section of the alter table reference pages includes the following bulleted description:
The partition number_of_partition and unpartition clause are provided for compatibility with versions of Adaptive Server earlier than 15.0. You can use partition number_of_partition clause only on unpartitioned tables to add (number_of_partition-1) empty round-robin partitions; existing data is placed on the first partition, with subsequent data distributed among all partitions. If the table has a global clustered index, Adaptive Server places subsequent data rows in the first partition. To redistribute the data, drop and re-create the index.
