The index you use to reorg a table determines the clustering of the table. If the most used index is a single column unique index, then that is often not a good candidate for clustering, even if it is the most used index.
Clustering has been discussed at length in other threads in this forum, with some examples and rules of thumb about choosing the clustering.
Clustering can also be designated by defining one of the indexes as the clustering index, in which case no index need be specified during the table reorg. If a clustering index has been defined, and an index is specified for the table reorg, then it must be the same as the clustering index.